One of the primary reasons — and it’s a perfectly legitimate reason — that people give for not finishing a book or a writing project is that they don’t have the time to write.
The good news is that at least half of the writing and plotting process takes place inside the mind, and you can do a great deal of important work when you’re driving, exercising, bathing, lying in bed, walking, gardening, et cetera. That process is called thinking.
Thinking must be approached systematically and with focus (e.g. “What are you going to do now?” “I’m going to think for a while.”)
In writing a story — particularly a long story — you’re presented with innumerable details and innumerable problems all of which you must sort out and solve. Many if not most of these are best done not when you’re sitting at your keyboard or over your manuscript with pen-and-paper, but when you’re alone with your own thoughts.
It may sound formulaic and overly systematic, and it may at first feel as if you’re bleeding the romance out of the writing process, but please hear me on this: writing a story is like working out a puzzle — a puzzle of your own devising — and the sooner you learn to approach it in this way, the sooner you’ll be able to finish your book.