Thank you, everyone! You're the best!
I just got back, and I can't feel bad about my performance at all. I feel that, if I don't get the job, it comes down to it not being meant to be. I'm happy with my questions, answers, and ability to talk fantasy books and caffeine-free beverages with my interviewers... In fact, if I weren't so jaded with the job market, I would be phsyched.
At least the hard part is over now - I hadn't known it would be a two-part interview!