If the "nerves" that you describe are things like shaking hands, sweating, rapid heartrate, and racing thoughts then these are caused by the release of chemicals in your body in response to a "perceived stress" (i.e. fight or flight). No amount of logical thinking will reduce these symptoms since these reactions are caused by chemicals (release of norepinephrine).
Therefore, when you feel nervous, do not try to fight it. Just allow your hands to shake, heart to race, etc. and realize that it is a biochemical reaction (not a character flaw). YOU CAN STILL PLAY WELL even while experiencing these symptoms. The important things to remember are 1) expect to experience the nervous symptoms, 2) "allow" the symptoms to occur (your hands to shake, etc) while remembering you can still play well with them, and 3) when you make a mistake don't turn inward with your thinking because then you are no longer concentrating on the music and will make more mistakes which escalates the nervous symptoms. Instead tell yourself over and over to "focus on the music". You can actually practice this at home by imagining yourself in the situation and concentrating on everything you think you will be feeling...
Good luck,
atticus