Focus on the music, not on the room, or the people in the room, or anything else. If you're playing with a piano, focus on how you are interacting with the piano. Pick something in the room to focus on while you aren't playing, a light fixture, or some fixed stationary object, NOT a person! Stay in the moment, block out the "what if's". Focus on your breathing, getting good deep breaths, especially before you start. Take two or three deep slow breaths before you start. Vusualize success, as in what will it take for you to feel that you are successful and then work to accomplish that vision.