I feel for you. We all choke at times, it's actually a good sign, it shows that you care, and it is a good first step to becoming a better player.
Here's some things that have helped me.
Focus on keeping your butt down after you hit every stroke.
Focus on using your legs to hit the ball.
You can't control alot of things in a tennis match and getting into thinking about stoke technique or even fancy strategy, leads many people (myself included) into being paralized.
By focusing on simple, little boring things like keeping your butt down you will have a positive way to expend your nervous energy that will have a good effect on you.
Go get em! Your gonna be a good player!