Loads of people (85%) won’t finish the book they start writing. I feel their pain. I had a couple of manuscripts biding their time until I found a method that helped me:
Write your first draft to your computer or your book—Don’t Tell Anyone about your story. No one. It’ll drive you crazy, but keep your mouth shut until the first draft’s completed.
Not talking about my work until I finished, meant I had to finish, and that gave me the energy to keep writing and finish the manuscript.