I have these problems, too, but I am autistic in a way that presents somewhat like ADHD. I eventually learned to have three rotating projects, so that when I get bored with one I can pick up the next thing until one gets finished and is replaced by the next project. The secret being, of course, to not let myself have more than three projects at a time or I will never get any of them done, lol.

Have you read about Save The Cat or Dramatica as ways to write out major plot points? My stories got way easier to write when I started using dramatic structure. A friend of mine pointed me to a podcast that talked about writing skills that covered lots of this stuff. Highly recommend! (Also they’re just plain fun to listen to.)