You wanna hear something really messed up? This one time, in band camp...(sorry couldn't resist) I had a week to memorize 5 songs to march in front of 25-30 thousand people in Tampa Stadium (before Raymond James Stadium, hell they still wore orange!) and I didn't have a single nervous issue with that. Not even preperformance jitters. I didn't think about it. But when I first started gigging out in clubs, I would practically have to puke before I went on. I think that was because I thought about it.
So quit thinking about messing up, and just go for it. What's the worst thing that can happen? Anyway you get used to it eventually, but it comes quicker if you don't think, and just do.