I had this question land in my in-box, and it, plus the asker's response, made me decide to repost my answer.
Question (paraphrased to keep it short) "How do you decide what story idea to follow through on? I have so many ideas, but I never seem to be able to settle on one to write all the way through."
I'm really not sure how to answer your question (and I've been thinking about it for a few days now). You settle on an idea the same way you choose the clothes that you put on in the morning: you say "hrm, that works" and you put them on, and unless something happens during the day, you wear them all day until you take them off. Or, in the case of a story, you write it until it's done, or you decide, for whatever reason, that it doesn't work the way you thought it would, and you either rework it until it does work, or you shelve it.
Sometimes you write a project because someone's going to pay you for it. If I had to choose between writing the next book under contract and a new idea, I'd write the book under contract because that's my job. I'd keep the new shiny idea until I had time to develop it (occasionally this leads to me writing several books at once. I don't recommend that).
Coming up with ideas, as the saying goes, is the easy part. Sitting down to write it, start to finish, is what makes you a professional. As you've discovered, it ain't easy...
The response (again paraphrased): "Thanks for trying. I guess I'll keep asking other people until someone tells me something useful."
People, just as there is no secret handshake that will get you published, there is no secret push that will get your book written. It's AiC all the way down. Pick your shot, follow through, start at the beginning and end at the end, and that's how you write a book. No other damned way. If you get distracted by a shinier idea, over and over again, then you're not a writer, because You're. Not. Writing.
I'm sorry if that wasn't the answer someone was looking for. It's the only answer I know.
- There is No Secret Push