Don't think, "Everybody is looking at me and this performance is so important that I can't mess it up."
Don't think, "I'm doing great, I feel rested, well prepared, and I'm going to play without mistakes."
Don't think, "I just played this wrong note which made a bad impression in the listener's minds. My performance is ruined."
Don't think, "Only one page is left in this piece. So far so good. I will finish strong."
Don't think, "My family is listening to me. I wonder what are they thinking now?"
Don't think, "My priest didn't show up today to listen to me play. He probably doesn't even care."
Don't think, "I can see this more educated and more experienced colleague in the audience who usually is very critical of my playing. Too bad that I announced this performance publicly. Otherwise he couldn't have found out about it."
Don't think, "I'm afraid. There are a few difficult spots in my program which I haven't completely mastered. I'm going to be punished for that by making mistakes in these places."
Don't think, "Next time I will learn from my mistakes and thoroughly master my pieces so as to be completely secure during my next performance."
Don't think, "Am I making progress? Is this performance will be better than the last?"
Don't think, "Is this will be better than Olivier Latry's playing?"
Don't think, "How am I going to improvise this chorale prelude?"
Don't think, "How am I going to improvise non-stop for 60 minutes?"
Here are two things to think about during your performance:
For the elements, notice rhythm, melody, and harmony. For the timing, do this for the current measure you are playing right now.
In other words, keep counting the parts of the current measure, sing each line, and notice any syncopations, dissonances, cadences, and modulations. This will also ensure that your breathing is controlled, too.
Think about the things you can control (elements and timing). Everything else - your insecurities, results, mistakes, and other people's reactions are not in your power to control. That's what the best musicians, actors, athletes, public speakers and other performers do (in their own way).
This is called professional attitude.