Another yet-unpublished "author" here, still trying to finish the same book I keep writing, scrapping, and rewriting since I was 12. (I'm 28 now, haha.) For those interested in knowing, it follows a young soldier in Company F of the 26th North Carolina through the war and into reconstruction, but I keep scrapping it and starting over. Part of it is I've grown a lot as a person and a writer since age 12, but part of it is also that I keep changing up what I want to happen, ie, what events the main character will experience, and when, in regards to non-historical events. Not even sure if I want to do it all as one book, or as a series. If it's one book, it'll probably be ridiculously long.
I can say that coming up with a "Plots and Fragments list" has helped a lot. It enables me to keep writing without forgetting the interesting tidbits I want to add, and preserves what I call "spinoff ideas," ie, possible new stories or alternate routes.
The one thing I have a hard time with is that there's so much interesting stuff that ties into the research part of the book, and I end up spending 99.9 percent of my time "diving into rabbit holes." I want the actions of the 26th to be as historically accurate as possible, even though my main character manages to find his way into some crazy messes.