Here's the only thing you know. You are the best trumpet player that's standing in the spot you're standing. It's OK to be a little nervous when you start playing. The ears need to adjust to the room, you have to adjust your sound to the audience, you have to adjust your ears to listen to the other musicians yadayadayada. I've been performing for decades and i still sweat alot for the first 20 minutes because I'm dealing with the unknown. However, once things get adjusted(like my ears), I'm OK. The best thing to do is understand that you should be a little nervous in the beginning. To cure this, work on getting your ears acclimated to the room. Once you get your ears in sync. You can close your eyes and begin to have fun with the music.