Allow yourself to be terrible. There are some writers who can create really excellent books in a first draft, but that's a rare skill.
I watched a movie with Greg Kinnear recently (Stuck in Love) where he played an author. He said something like, "I'm a terrible writer but an excellent re-writer." I think that's probably true for most of us.
Get the book down even if it's the most awful thing you've ever done, and then go back and fix it. Most of my books have gone through at least ten different types of revisions/rewrites before they reach the public eye.