After three months of being unemployed from a direct hire permanent position, one of my new co-workers at this great company just mentioned that interviewing a candidate should be like talking to an old friend.
Take that to heart. Not all interviews will result in a job for a million possible reasons not related to you. It could be anything from the guy got cut-off on the way into work to him or her getting yelled at by his boss that morning. If you don't get offered the job, chin-up and do what I did; Treat it as "interview experience". Don't kick yourself too much. Relax. Be yourself. If they don't like you enough to hire you then you probably won't like the job anyways!