Try to use visualization thecnichs! Just imagine your perfect show, without fails, without nervouls, whitout anxiety...everything is going to be all right!!! You prepare your mind in a positive way for success!!
I like to try and focus on the very next task i have. If i find that im overthinking things again i try to sit back, if time allows, and breath and visualize my course. Focus on one thing at a time, too much in your head can cloud your judgment. Make sure to hydrate! That can really calm you down.