I think part of the problem is that I'm remixing a fic for an author whose fic I *really* like. I didn't think this would be a problem, until I find myself sitting and staring at her wonderful opuses (opii?), and wibbling about how to rewrite things that are so obviously perfect. And even now that I've started, I'm obsessively going back and checking every detail, to make sure I'm not messing up (her) canon, and I think my fic is flat and uninteresting, and aarghh!
I suppose it doesn't help that I've picked a 15,000 word time-travelling epic to remix though, does it? :)