Can you offer any advice for avoiding hoarding when part of the problem is that trying to deal with the clutter and garbage and dirt causes paralyzing anxiety? I want my house to be clean and cluttered because it's stuff I like, but instead it's full of trash and stuff that had a place but doesn't seem to fit back in it after being used.
I can absolutely offer advice about that.
Select the room you want to clean and make a map of it.
Divide the room into small segments like "top of desk" or "cabinet under sink" or even "half of junk drawer." SMALL segments.
Designate bags "trash," "donate," and "consider later."
Schedule a time to work on cleaning each segment, don't just assume "i'll do it next week." Write down an assigned day for each area.
Go into your target area and sort things into those bags.
Optionally, create a bag for memento items to put into a specific memento box/book.
Take bags out of the space when they are full to make more room to work and to see progress.
Do the section for the day and stop. Don't get overwhelmed by a ton of stuff, stop when you've done what you planned for the day (unless you've got good momentum built up and continuing will energize you.)
Go someplace where you are not looking at the mess. You want to draw a map of the room, but you do not want to be in the room. Work one room at a time.
Divide the area you want to clean into very small spaces. You aren't cleaning an entire desk, you are cleaning one drawer of a desk.
Take three containers with you for each section: one trash bag, one donation bag, and one bag of stuff to consider later.
Plan out time to work on the space. Don't say "I'll do the whole thing this weekend" or "I'll get to it after the holidays," sit down and write out a schedule. There's a version of this called 40 bags in 40 days that people do for lent (that was the version of this i first found and followed the first time i did it), but you could do it in ten days, or a hundred, just try to stick to working on each segment on the day it's scheduled.
In each space, keep the stuff that's obviously meant to go there in that space, so if you're cleaning a desk drawer and it has a stapler in it, the stapler can stay there but if the staples and paper clips and rubber bands are a mess put that stuff into the "consider later" bin. Same thing with papers; if you've got a bunch of papers and you may need to keep some and may need to trash some, put them in the "consider later"
THERE IS AN OPTIONAL BIN FOR PEOPLE WHO WANT TO HANG ON TO A MILLION MEMENTOS AND CONCERT TICKETS AND SUCH. I make them by getting gallon freezer bags and filling them up with business cards and concert programs and scraps of wrapping paper and birthday cards. This isn't quite "consider later" because it's probably stuff you know you want to keep, this is "I don't have a home for this thing right now but it's not trash" so this is a temporary home for that category.
Remove stuff from the space as you work. As you fill up a bag of trash or consider later or donate, take it out of the space so you aren't looking at it and you can see the progress you're making on the space.
Do each section as you come to it on your schedule and then call it quits. If you cleaned out the counter next to the sink and that was your area for the day, you don't have to worry about the area under the sink unless you have the energy and enthusiasm for it.
Philosophical musing about why this works
The reason this kind of plan works (for me) is by pre-managing several things. You know you're working with a limited area, you know what you're going to do with the stuff you find in that area (put it in one of your bags or leave it where it is if it belongs in that area), you're working on a limited time so this can't stretch out forever it's just a little chunk, you're thinking about the space as you build your plan so you're visualizing the anxiety inducing thing outside of the space that actually gives you the anxiety which hopefully allows you to detach slightly from the anxiety, and you're getting your steps lined up ahead of time so there's no muddle of "what do i do now, how do I get started" - you get started by grabbing your bags and you go to that day's scheduled section.
The whole thing is constructed to prevent you from getting overwhelmed.
I used to try to clean my room as a kid and I would find something that needed to get put away but I didn't know where it went so I'd spend a bunch of time trying to make a space for it and I'd end up getting lost in the weeds of imagining how I'd use the item and if the new place for it was accessible, and oh look at the items that I found in this other place where I was going to put this item and this method cuts off all of that. Where I am putting the item is in the bag, where it is going is the "consider later" pile and when I've cleared out most of the space I can consider where things go when I've gathered all the uncertain things into one place instead of continually unearthing them and disrupting the process of going through stuff.
What it means to Consider Later
The reason you're working room by room is because you should be isolating the consider later pile by room. If you're cleaning out the bedroom you may end up with stuff that belongs in the kitchen or the office, but you'll end up with a lot of stuff that belongs in the bedroom. When you've worked through all your segments, you can sort the consider later pile and now that you have all the objects together, you can consider whether some of them belong together in a space in the room.
For instance, when I first did this there were a lot of books that needed to go on bookshelves, but my bookshelves weren't accessible in the early parts of the process. So books from the floor and the bed and the nightstand went into the consider later pile and after the whole floor was clear and there was no trash on my desk and all the books I was donating had been pulled from my bookshelves, I was able to organize all of my books at once instead of stumbling across a book every four minutes and trying to shelve it.
That's what spawned the memento bags for me; there was a ton of stuff in my consider later bags that didn't precisely have a place but weren't trash and needed a place made for them. If I'd struggled to find where each item went as I cleaned it would have completely stalled me out.
I kept finding yarn as I went but I didn't have a dedicated yarn spot, so I just put yarn in the consider later pile and at the end I found a basket for it and put it on a shelf in the closet that had been cleared out when I'd donated old clothes. If I had tried to find a spot for the yarn before donating the clothes, I would have had to move it once the better spot opened up, so saving all the consider later stuff for later saved me from having to move stuff several times.
If you're in a small space or if you're living with people and you can't make a pile of stuff in another room for two weeks, at the very least remove the trash and donation bags as you go and designate an area for your consider later pile; maybe a laundry basket or something similar so that you can keep it mobile as you clean.
It's kind of like moving in to a new space. When you move in to an empty room, you have all your stuff in boxes and you need to figure out where it goes and that can take a while, but it's sometimes easier to find a place to put things in a new environment than it is to put things back "where they belong" because maybe you've added a dozen skeins to your collection and they don't belong in the little yarn bag anymore.
What to trash, what to donate, and what to consider later
Trash should be immediately obvious as trash. Anything that is trash goes in the trash bag right away.
If you find yourself thinking "but I might use this plastic fork that came with my value meal," or "this receipt may be important," put it in the consider later pile and don't think about it right now.
The donate bag should be for stuff that will still be useful for someone, but won't be useful for you. Clothes that you don't like, books you hated and won't re-read, toys you don't want to keep, all of that goes in the donate pile. If you think you might want to keep a piece of clothing but you want to make sure it doesn't fit, don't stop to try it on now just put it in the consider later pile and you can sort it into the donate bag later.
"Consider later" is for anything that requires more than thirty seconds of thought or effort to handle. If you're looking at your desk and you've got a keyboard for your computer on your desk that keyboard is staying there and doesn't need to be considered. If there's an empty takeout cup on your desk, that cup is going in the trash and doesn't need to be considered. If there's a receipt for your computer sitting on your desk, you may want to save that for record-keeping purposes but may not have a place to put it, so that is what you consider later.
Some guidelines on what is or is not trash
You might look at a sturdy plastic cup from a gas station and say "that isn't trash, I could use that, that's still good" but unless you have a specific purpose in mind for it right now, that is trash. If you wouldn't put it in a donation box to be used for some ambiguous future purpose, you don't need to keep it.
If you have a specific purpose in mind, like using an old milk jug to make a watering pitcher for your plants, it may not be trash. But only ONE is not trash; more than that is trash.
If you wouldn't need to have a hard copy of a paper and you have an electronic copy, it is trash. This means receipts for most everyday purchases like groceries and fast food. Don't keep receipts for items past their return period, don't keep receipts for items that you have a digital copy of unless that item cost over $1000.
Nice cardboard boxes (or good glass jars, or sturdy plastic takeout boxes, or cleaned food containers) that you don't have a use for are trash (or recycling, depending on where you live, but still in the trash category).
If you know someone who is specifically looking for an item (like maybe the neighbor kids are asking for cardboard tubes for a science project, or you work with a meal delivery group that could use extra packets of takeout utensils, or you have a friend who is into canning and has asked for jars, or if you make your own soup stock and need containers to put it in, or if you have a friend who is moving and needs lots of good cardboard boxes) then these items don't *have* to be trash but if you are just keeping them in your space and not giving them to people who want them or putting them to use yourself, they are just trash in your space and you should throw them away.
Memory Books/Memento Bags
I make memory books out of the little items i collect into one gallon storage bags. They allow me to hang onto the stuff that I want to keep because it brings me good memories without having a pile of random junk and sometimes without having to keep the item, or having to keep the whole item.
If the thing I want to keep because it brings me good memories is bulky, perhaps I can take a put a picture of that item to put in the book. If it is a worn out shirt, perhaps I can cut a patch off the shirt to put it in the book. If it is a card, perhaps I can cut out just the front of the card, or I can almost certainly just throw away the envelope and put the card in the book.
If you have things that do *not* fit into the memory book, like costume jewelry or rocks or a weird toy you got out of a coin machine on a really fun family vacation, you can also make a memory box; I have some of these and they've got a bunch of truly random crap in them, but I *like* having the nametag from the four hours that I worked at Denny's, or the keychain from when my mom took me to the morgue training class. It's fine to like these things, and to keep many of them, but you want to keep them someplace that they won't stress you out; that might be a display case for nice things, but it also might be a pretty velvet bag that you periodically pull out of a drawer and sort through like a magpie, or a wooden box that you painted.
You can also be selective about this stuff. You don't need every piece of costume jewelry your grandmother owned; keep the pieces you really like or the ones you have strong memories of or the ones that are very nice or the ones that are in good shape. But look, my mom was a teacher and she had a wide variety of goofy holiday jewelry that she wore in the classroom and I don't need to hang onto that. I don't need the big plastic ghost earrings that won't fit in my plugs, but I'll hang onto the spider brooch. She collected cheap watches - I don't need all of her four dollar watches, I can keep the nice ones, or the one that she got for ten years at her job. Do the same thing with stuffed animals and baby clothes and magazines and children's books. You don't need to keep all of it, and keeping all of it isn't going to help you remember that time more, or remember that person better.
Do you really want to keep it or do you feel obligated?
Youtuber Caroline Winkler (who has some great videos about home organization that I like a lot, in particular "this is why your home is a mess" - with the caveat that she likes closed storage and my ADHD ass loves open storage) has a really great tip on getting rid of stuff that works a LOT better for me than the Marie Kondo "Does this spark joy?" question and it's the Red Wine Test. Instead of asking if an item sparks joy, you ask yourself "If a bottle of red wine spilled on this (or if it was in some other way damaged) how hard would I try to fix it?" If you wouldn't try very hard, or if you would be *relieved* then you can get rid of that item. If one of the Venom mugs I have on the shelf fell down and broke, I wouldn't try hard to fix it. If my cat stuffed animal from when I was a kid tore open, I would immediately be looking for my sewing kit.
.... I should recycle those cheap teal glasses, actually.
Some general tips that may help to get you started that work for me and my ADHD and may work for you and your anxiety:
Start a timer for a short time. You don't have to clean your whole house, you are just going to pick up for five minutes. Then you can stop, and you only have to face a *little* bit of the anxiety.
5-4-3-2-1-go. Don't overthink it, count down quickly and then get up and do something. Keep going in as long a spurt as you can manage without getting too upset, but cutting down on the time for pre-game fretting might help with the anxiety.
Do the smallest amount possible. You don't have to clean this room, you just have to take one dish to the sink. You don't have to do all the dishes, you can just unload part of the top tray of the dishwasher.
Some general tips on trying to keep a space clean:
First, encouragement: It is a lot easier to maintain a clean space than it is to create one.
If you're thinking that something needs to be done and it can take you under five minutes to do it and it's right in front of you, do it. I do this with my dishwasher. It turns out unloading the dishwasher is the main thing that stalls me on dishes and keeps my sink full, so now when I'm waiting for the kettle or letting my tea steep, I unload whatever I can get done in that time. If I have the vacuum out and I did my living room but the hall and the bedroom could use a quick pass too, I vacuum them while I've got the machine in my hand.
Set success traps. Success traps are things that let you fall into succeeding by front-loading the effort (or executive function) of cleaning with planning. Trash collects in your living space? Put a bunch of little trash cans everywhere. Cleaning your bathroom takes extra time because you have to go get glass cleaner and paper towels from another room? Keep a bottle of glass cleaner and a roll of paper towels under the sink. You never sweep because it is a pain in the ass to get the broom out of the broom closet? Hang the broom from a mount in the kitchen. It takes too long to clean the counter because you have to pick up a bunch of makeup brushes and bottles and soap? Put that shit on a tray and now you only have to move one thing to clean the counter.
And for your specific question, with "things never seem to quite fit back where they came from" sounds like you're playing storage tetris, which is when things have a place and it is a *very specific and exact* place that doesn't have a lot of room around it. You may need to think about downsizing for your space, or, more likely, think about more efficient storage. That Caroline Winkler video I linked has some tips on this ("don't store things in a way that will make you angry like putting your common use objects on an out of reach shelf or you'll never put things back because it's hard to put them back" and "maximize your weirdo spaces" speak to your situation, i think) that I've put into use, particularly in my kitchen. It was hard to keep the counter clear because it was hard to put my stand mixer away because the rack for the stand mixer had a wok and a bunch of cast iron pans and a panini press and a chafing dish on it; I put the panini press and the least-used cast iron and the chafing dish and the wok in a more out-of-the way cabinet (because i basically never use them but they're very useful when I need them) and now that shelf has a little grill, my more commonly used cast iron, and my stand mixer so putting away the stand mixer is a lot less effort so my counter stays clear. I wasn't using the top shelf of my dish cabinet for dishes because it's too high up for daily use, but it's perfect for the rice cooker, waffle maker, and food processor that I use less than my dishes but more than my george forman grill.
And anyway, the TL;DR for all of that:
Work a little bit at a time, be nice to yourself, don't keep things that aren't worth keeping, and configure your storage in a way that works for you (by keeping your lifestyle, the way you use things, and how easy it is to put away into account before deciding that's where something lives).