Don't fight it, use it.
Anxiety and stress actually increase your physical abilities (faster heart rate, increased glucose levels in blood etc...)
The trick is to learn to keep your mind cool while your body is upset (shaking knees, urge to pee).
On a psychological point of view, the fear to be affraid is worst than the fear itself, but you can come to see it as a free and legal hormon boost. Studies showed that stress can increase your performance level more than illegal drugs likes amphetamines.
I had similar issues as a teenager, and used to perform badly when the competitions where of major importance : well prepared, perfect physical condition, but the stress made me unable to think right, and I made bad choices. you know, the one that you ask yourself after : why did I do that.
I tried relaxation (sophrologie), before to realise it did me no good to be without stress during event : I made me less performing, physically.
So I worked hard to use my stress instead of trying to limit it, welcoming it as a mesure to my motivation.
It's very hard to explain, but it's possible to disconnect your emotions and your reasoning, not unlike ridding a wild stallion ;-)
Side note : I never meet top level competitor immune to anxiety, it's just like courage, you don't know what it is if you can't feel fear.