Take some time the next few days to practice some relaxation techniques, even it's just some basic stretching and light breathing. If you're starting to get really anxious in the interview, wait for the interviewer to ask a question, then say, "That's a good question. Let me think about that." Then while you've got a second, take one or two nice deep breaths and proceed. And remember: no matter what happens, it's not the end of the world.