Some days I just don't feel like writing. I don't want to get on with whatever story I'm writing because I know it's bad, I'm not sure what to write next and I'll just mess it up if I try.
Then I found Writer's Resources by Timothy Hallinan:
The enemy is not the badly written page; it is the empty page.
If there’s one rule you should write on a card and tape over your desk, this is it. A bad page does a lot of good things: it advances the story, it gives you a chance to work with your characters, it demands that you write all or part of a scene, it challenges you to describe your setting – on and on and on. (It even makes the stack of pages look a little thicker, which can give you a psychological lift.) So what if it does some of these things badly? You’ve learned one way not to handle that particular piece of material.
But the great advantage of a badly written page is that it can be rewritten. It can be improved. A blank page is zero. In fact, it’s worse than zero, because it represents territory you’re afraid, unwilling, or too lazy to explore. Avoid exploring this territory long enough, and you’ll abandon your book.
You'll have to excuse me now. I need to get back to writing my story.