This sort of happens to me a lot, actually. I usually fic when I become heavily invested in a fandom (or relapse into an old fandom I thought I'd abandoned), and I feel like I have to write something - carry out a what-if scenario, give the characters a happy ending - anything, to satisfy my ship or whatever.

But then during the writing process, I always quickly lose interest because the fic is already composed in my head - I feel like writing it all down becomes a hassle, because I've already salved my need to 'extend' on the canon, and I don't feel like anyone else would be interested in what I write (especially if it's an obscure fandom).

After reading your post, though, I've decided to commit myself to finishing an old WIP. So, er, thanks. :P