I had a pretty bad attitude and had trouble focusing on work. I thought it was depression, and that's probably a factor of it, but having been unemployed for the last three months now, I think I was just going through a really bad case of burnout. I didn't really want to do anything productive, I just felt bad, and as a result I was missing deadlines and not doing terribly well, and eventually they decided to let me go. They told me it was budget cuts, but I think that might have been more of a catalyst than a cause, and this was depressing because by all accounts this was the best job I'd ever had.
If anyone's reading this, take burnout (and depression) seriously. I lost a great job because I didn't.
[1] I'm reasonably confident you could figure out which company I'm talking about with some searching, but I politely ask that you don't post it here if you do.