I was wondering if you could make another quick rec list for the like... body horror/alter ego/monster mangas you've read? For example: Dawn Tsumetai Te, Tokyo Ghoul, Ajin, Iron Knight, etc. Know what I mean? I've read all those so if you can't think of any other ones it's fine. c: Sorry to bother you.
You named all my favorites other than Blue Exorcist or Deadman Wonderland yep. Some of these I haven’t read in years so I might be going off feelings of memories for some.
666 Satan - Masashi Kishimoto’s twin brother made his own shounen adventure, and honestly I prefer it a million times over. Instead of a demon fox, Jio has Satan sealed inside of him. Also known in America as, O-Parts Hunter.
7 Billion Needles - aliens and body horror
Alive - The Final Evolution - Evolution makes you different from the rest of the world, although that’s not what you should be worrying about.
Antimagia - Beautiful art and story. It’s been finished for 2 years but the very last chapter is not scanlated anywhere.
Aphorism - Not really monstrous, although there are monsters. It’s like an alter ego but it’s actually more complicated than that.
Apocalypse no Toride - My fav manga at the moment. Zombies and really messed up body horror mutations.
Blood Type V - super interesting vampire oneshot
Crimson Spell - demon curse yaoi with a plot
Dansai Bunri no Crime Edge - Mental rather than physical. Owning scissors that belonged to a serial killer is never a good thing. Not as explored as much as I would like.
Guardian Dog - more aliens but this one is more obvious
Imawabi no Dakini- Spirit possession that leads to servitude.
Kingdom of Zombie - FREAKING AMAZING! However, there won’t be any updates for a long while… One of my favs though. On the same level of love as Blue Exorcist if not higher.
Kure-nai- There was a certain part that reminded me of Dawn Tsumetai Te but I don’t remember it being as fully utilized as I was hoping. Maybe there’s more to it in the manga or light novel or I’m just remembering wrong.
Kurozakuro - Definitely in the group your looking for. Unfortunately, you can’t find the last 10 chapters anywhere…
Nurarihyon no Mago - Not really a lot of inner struggle in this one, but there is a sort of “alter ego”. Nura is 1/4 youkai and his appearance changes drastically between forms.
Pupa - Both siblings struggle with their humanity. One, a physical monster, the other, a mental monster. Haven’t heard a peep out of it since the sorry excuse of an anime came out…
Rosario+Vampire - ((NOT THE ANIME)) Tsukune has to deal with something crazy later in the series.
Tokage no Ou - Mental rather than physical. The manga’s only a very long prologue of the novel which I haven’t been able to find.
Variante - Adding it because it fits the bill. Tbh, I really liked the art and the beginning but the end made me hate the entire experience.
This is mostly for the alter ego/struggle from within type of list. For more body horror stuff, I look into more “classic” horror manga because it's more unnerving with that stuff than with more modern manga with pretty art styles. Stuff like Parasyte, Inugami, Emerging, or anything by Junji Ito.