Mike Tyson says something that rings so true for music: everyone has a game plan until they get hit.
We practice our pieces until they are perfect, developing a gameplan. Then the performance happens and we get hit by a memory slip or flubbed passage. The game plan is out the window. How do we prepare for that?
Perhaps we need to get into the ring more often and get used to getting hit. Once we realize that a bad note does not ruin a performance we can keep the gameplan in our heads amidst the pressure.
But again, this requires us to get out and perform. I’ve learned that performance is a muscle. You have to exercise it in order for it to reach its peak potential.