#ugh... i can't self-advertise to save my life... tags from @rubytactician
I have a trick for this! It's called ask someone else.
No, really. Someone in your friends / family / favorite colleagues from that last job is going to be your biggest supporter, so make use of that!
Ask them how they'd pitch you to someone else. Insert theoretical scenario of them somehow socially encountering someone hiring for the perfect job for you - what would they say to make that person interested? What about your work history do they think would be compelling? What skills do they think you should promote?
It is really hard to figure out how to market yourself. Especially these days when there's a more performative aspect to job applications than just having the right skills. Make use of the people who love you want to see you do well.
I think this sort of thing used to be easier when written letters of recommendation were more common, honestly, because those were literally someone else advertising you to someone else.
Another thing you can do - which will feel so awkward but is worth it - is to ask friends, colleagues from previous jobs (maybe not your current one), or past/current clients to give you a blurb! If you have a professional website, those are a great thing to use to flesh them out to prove you are a real person with real skills. If not, you can keep them to look at when you need inspiration or can't remember to save your life what you're good at and why someone should hire you.
My best suggestion if you're like uh, no, I cannot write something cogent is... don't. Barf everything vaguely relevant onto a sheet of paper. Do a free-writing exercise with no constraints. Write down all the silly, ridiculous, egotistic shit you'd like to tell an employer about why they should hire you. Get it all out on paper without censoring yourself so you can get any of it on paper - and this this important, do not send it. Read it back, and draw from it to make your actual cover letter. If you're struggling, show it to someone else and ask them what they think is important/interesting/marketable.
Drafting things like cover letters and even your resume with the support of your community will seriously make them so much better. You get eyes on for spellchecking and grammar, but also to help make sure it flows and that you're not using jargon or technical language a hiring manager might not understand. It can be hard to ask and I totally get that. It's something where I think it's worthwhile to sit with the discomfort. Applying for jobs successfully really isn't a skill that's taught these days! So we succeed best when we support each other.
I practice what I preach here. This is hard for me too - I am successful because I lean on community. I don't remember the last time I wrote a cover letter without having someone else preview it, even though I have a solid template I've worked from for years. I literally just asked @kaijutegu to help me cobble together a bio for a client that wanted to include me on their website (aaaaaaaa). I read and give feedback on cover letters for my friends as well, because it's got to be a two way street.
I won't say that writing cover letters isn't hard, because it is, but it is a skill that can be learned. And you don't have to do it alone.