Call it what you want - performance jitters, butterflies, stage fright - it all comes down to the same sickening feeling people get before performing. It doesn't happen all the time but when it does, it can be paralyzing. Here are some interesting articles on how to overcome stage fright (some are music related, some are general):
Also, I hear these books are very good:
The Inner Game of Music by W. Timothy Gallwey
The Young Musician's Survival Guide: Tips from Teens & Pros by Amy Nathan
Happy reading.
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