Here's something I found on Kristin Cashore's blog - she is the author of Graceling and one of the illustrious Dave Cornish's fellow nominees for a Nebula. It's quite a relief to know what I'm feeling is apparently normal. I could have written this myself (except for the girly way it ends - I would have ended by saying 'bugger off where you came from, worried thought!').
I worry constantly about whatever book I'm currently writing. I worry about the wording, I worry about the themes, the plot as a whole, whether the characters seem to others the way they seem to me, whether the book is getting too long, whether my protagonist is likable, whether my fantasy world is consistent, whether I'll be able to hold everything together, whether there's even anything worth holding. There is never a moment when I don't have something to worry me. I have learned that this is just what it feels like to write a book. Most of the time, I can keep it from bothering me. You get good at ignoring the voices. Or giving them the attention that's best for them: listening to them and laughing and giving them a hug, and saying, "Yes, I know you're worried. It's okay. Let's go watch a pretty sunset."