everytime a bad or nervous thought comes in to your head completely shove it out and change it, even if you dont believe it, to a positive thought. i,e AAH I AM SO NERVOUS I AM GOING TO MESS UP I KNOW IT ===>> I AM FEELING CALM AND I WILL DO MY BEST
the only difference between you and someone who doesn't get nervous is your thoughts, those who don't get nervous do not allow themselves to think ahh im scared etc you have to take control of your thoughts and completely blank out the usual, also dont over think.
Hope I helped, i used this technique to help me get over my fear of needles and my anxiety in exams