I've always had severe performance anxiety. IN high school sports I would get sick to my stomach before football/basketball games. In various bands I would feel the same before we would go on stage, and public speaking in college. Finally I just said...fuck it. Worrying has never done anything for me. It has not ever improved the situation. When you start to realize what you are afraid of you can find ways to combat the nervousness. For me it was fear of looking stupid...which is a dumb thing to worry about anyway.
Everyone gets nervous, even at the highest levels. But the mechanisms they use to deal with that nervousness is what matters. For some it is relaxation, for others it is staying busy with anything they can to avoid thinking too much...for some it is reading or listening to music, for others it is all about visualization of the outcome. You have to find what works for you.