I generally have three responses to stage fright.
First, think of times past when you've sat through mediocre or worse performances by others. Now, it's your turn to do it to them. There's no down side, in fact, you may surprise everyone with an entertaining performance.
Second, most of what you hear in your head, and what causes you your initial anxiety, is the fact that your tone may waiver or your timing might start off different than when you practice. But the audience isn't familiar with your "regular" tone and timing. This is what freaks people out about public speaking. You can tell in your head that your voice sounds different because of nerves and it snowballs from there. But your audience doesn't know whether your voice always sounds like that. Of course when you play you want to get in your groove because it's a comfortable place, but the audience doesn't know what your regular groove is, so you have time to find it. This is probably the most calming thoughts for me. Instead of worrying about a squeak I made in the last measure, I'm busy searching for that comfort zone where I can show what I know.
Third, public playing is simply reality. Practicing at home, or with a teacher or group that you are familiar with, can be too relaxed. Public playing is a reality check. Unless you've gambled your life, job, car, house, 401K, etc., on the outcome of the performance, there's no down side and you will learn a lot.