Let go of the idea that you will write things in order. Let go of the notion that you will create anything without multiple drafts. Writing is one of the messiest fucking creative processes out there, and every writer does it in a different way. The few stories I've actually finished bore little resemblance to the first draft once I was finally done with them. Just write. Produce content. In the past year I've been inconsistent but I've filled more than one notebook almost completely with my story ideas, characters, outlines, bits of writing, sketches, lists of places and technologies, etc. and started to form a coherent narrative out of the ideas I've had floating around in my head.

Coming back to your ideas about writing: when you read a published book, you're looking at the end result of years of brainstorming, prewriting, drafts, rewrites, edits, notes, large sections being cut out or moved around, etc. It's a process. It takes a while. It's frustrating. It's painful. It requires lots of doing the same shit over and over again. As far as I can tell, thinking you're a talentless hack who will never succeed in anything or even finish this damn project is normal, it's a part of the process. Just like failure. Failure is necessary. I'll repeat that: Failure is necessary. Part of learning what parts of your stories work is learning what parts don't work. Sometimes you have to throw shit away. Sometimes you go digging through your own trash, though, and find an idea or a bit of dialogue or prose that you can reuse somewhere else, or jam together with another thing that didn't work to make them both work.

In short, just start writing shit down without rhyme or reason. Something will form, layer by layer. From there it's just the usual motivational shit and organizational shit that every other writer has to deal with, just with that extra layer of neurospiciness on top. Take your meds, do your stims, get your dopamine flowing, and believe in yourself. Be your own biggest fan.