"Only those who dare to fail greatly can ever achieve greatly." - Robert F. Kennedy.
I've had the same thing happen to me--twice! Both times I was humiliated beyond what I thought I could handle, but both times I came back and grew from it as a person and musician. Three things that come to mind as actual advice for you are:
1. If you didn't already do this, make sure that you know what you're going to play so well that you could do it without thinking about it. Don't expect to be able to improvise anything. Don't assume that your performance will be good; assume it will be only as good as your sloppiest practice the day before, so make sure your previous day's practice is excellent.
2. Control your environment as best as you can and never over-commit yourself to something that you aren't positive you can handle, unless you don't mind if it crashes and burns. I don't mean that you shouldn't take risks--not at all--but just be as realistic about the outcome as possible.
3. Learn how to deal with stage fright. It effects loads of people everywhere and there are definitely things you can learn to do to help you cope. The best things are practice, creative visualisation, proper diet that day, and deep breathing.
Oh, and have fun. Watch your attitude. Don't play to impress people; it will just put a ton of pressure on you. Do it for fun, for love, for the money--anything but to show off.
