Can you find anyone who can teach you basic relaxation methods? Things like Alexander Technique, yoga, meditation, general relaxation ... I've always found them very helpful. The thing to remember is that you need a relaxed focus, not a relaxed floppiness, and with practice you might be able to achieve a clear-headed focus that is enhanced by the adrenalin rush of performance ... that's the theory, anyway!
A quick google brings this up:
http://www.helpguide.org/mental/stress_rel..._relaxation.htmAnother technique is to ask "What's the worst that could happen?" - imagine you plough your exam, your hand falls off in the middle, the examiner has the interpersonal skills of the Incredible Hulk, the piano falls apart, you trip over and fall on your instrument (this actually happened to me in my Grade 2 clarinet) ... etc. And guess what - even if you failed with minus 10 out of 150, you'd survive! And the actual outcome will be much better than that, so ...
In the old ABRSM book about preparing for exams, there was the excellent line, "if all else fails, imagine the examiner in his underwear". Works for me!