If you’re a creative person (which, I assume, you are), you would sympathize with another when they say they have too many ideas to keep track. As a writer, my brain is constantly coming up with stories, any kind of story, even plots for possible future installments of favorite video game series. It becomes quite overwhelming. In one of my journals (and I forget which) there’s a list of novel ideas I would like to write, and somehow it reached past 30. I helped myself by highlighting the favorites, the ones I had a clear concept for, but even that was too many. At some point I decided to pick one project that I felt prepared for. It was important that I knew enough of the plot, characters, and elements to write it with confidence. I never like to write something that seems to slip out of my control (however there are the rare items that turn out better when that happens).
The more important thing, though, was making a commitment to that story. I have begun and not finished too many stories to let that happen again. I had to basically marry myself to it, and if I found myself working on another project, I was cheating on it. As someone who doesn’t like to multitask if I can help it, it was crucial that I remain focused. Besides, when i’m working on more than one story at a time they end up jumbling together and I mistake my protagonist’s name for another protagonist’s name (been there, done that).
If you have every intention to finish a project with flying colors (any kind of project; not just a book), it is imperative you commit yourself to it. Eliminate as much in your life as you can, the parts that are not priority, and put as much focus in it as you can. And don’t sell yourself short in that area. Be realistic, but don’t be withholding. It’s your passion. Treat it like it is.