Unless you’re Mr. Olympia, nobody expects to lift a 100 pound dumbbell on the first attempt. It’s something that has to be worked up to. Our muscles need to expand and grow in order to finally take on this enormous task.
We get frustrated that our first performance of a piece was pitiful, highlighting the fact that we practiced the piece so much leading up to it. But practice and performance are two separate things.
Performance is a muscle in itself. The only way we will get to performing better is by performing more, learning and growing from each experience. Soon enough we can lift 100 pounds, we can perform that piece at a high level in front of a bunch of people.
One caveat: as much as a muscle can grow strong it can also weaken due to lack of use and laziness. Performance is the same way. It needs to be constantly worked lest it weakens and we’re back to square one.