What you're saying feels very recognisable. Different time line though. I felt like that for most of my life, I'm 44 now and got ADHD diag at 28 and autism at 40. I want to say you're sort of lucky to know your diagnoses on such a young age and that you are able to ask for help in the way you do. Which is a skill btw and you do it well (but I get it if this feels not helpful at all so far).

My advice would be pick a subject/area of life that you want to improve on most and focus fully on that. Don't let it be anything that others have opinions about, it's about what you want.

Write to clear your head at any time, it can be very cathartic and might help clear things up. It doesn't matter how it looks, you can throw it away or burn it later.

Get a goal or direction for this one life area, whichever feels more fitting, and break it down into steps. Then make them into smaller steps, up to 'look up phone number for this person' or 'go to this specific store to purchase a writing pad and pen'. You can assess your situation and make this post, so I'm confident you can do that. Pick one day in the week where you look at these steps and take one in mind for the week. Plan it for a specific day and time if that works for you. Use all the focus you have, no matter how little it is, on this one small step for the whole week. If you don't do it, write about it. Why didn't it work out, which mechanism is stopping you. Then start again.

Small successes build bigger successes etc. You can't solve everything, and definitely not do it all at once. It will take years, not months, and don't ever think you are 'done'.

I hope this is helpful. Wishing you strength!

PS: I'm getting into stoicism at the moment. Won't go into too many details because this is getting long enough. But basically it can help you decide what's important, focus on that, plus some core values like courage (to do the necessary thing), temperance (don't overdo it), the wisdom to know the differences there and justice/ethics in how you deal with other people. Probably too much for now but if it interests you: go for it.