Dealing with Stress - for actors

Does anyone else think too much? I can't stop thinking about not being specific enough in my acting, how I feel like I suck all the time, singing and not being able to mix properly, and so forth. How do I deal with the fact that I just had a second bad audition in front of a guy who's been getting appointments for me, and I am now convinced he thinks I'm terrible and shouldn't waste his time on me?

How do you actors out there deal with yourselves, and with stress, and with worrying so much about everything?