I learned to conquer my doubting demon by recognizing that pieces of music have a certain life of their own. They're memes that self-assemble in your head, or in the collective heads of your collaborators. You may have a certain amount of influence over their early gestation, but you can't control the way they come out any more than you can control the way your kids grow up. The best you can do is to be nurturing, to intervene when necessary, to stay out of the way when necessary, and to try to appreciate the final shape that the music takes on for itself. Imposing time limits on yourself is a big help. If you force yourself to stop working on something after a week, or a day, or an hour, you'll be pleasantly surprised by how beneficial the results are for your creations.