I'm a writer rather than an artist, but I think I experience some similar frustration when I'm working on stories of my own. I know how I want the story to go in my head, but when I put pen to paper it comes out wrong somehow, or just not quite how I imagined it, and I end up scrapping it and starting over a million times.

I don't know how feasible this advice is for art, but I can share a lesson I've learned the hard way: Just put something on the paper. That's all you need to do. Even if you hate it, at least it's something. Once you've got something, you can work with it, mold it into the shape you want it to be, or at least something close. The temptation to edit your work as you create it is powerful, but it must be avoided at all costs. Perfection is the enemy of progress.

Like I said, I'm not sure how well that advice transfers over to drawing/visual art (or maybe I just completely misunderstood the issue in the first place, lol), but I hope that helped somehow 🙂

You should consider starting on smaller projects so you can get comfortable with the space and understand exactly what you want for the main project you’re passionate about. Starting by making really small projects is what indie game devs usually do and you could do it by starting with a really small manga and then maybe a really small indie animation