Performance Is NOT Perfection!
The dichotomy for musicians, particularly classically trained musicians is that we must strive for perfection, but remember we will never quite make it! Even if you can play a "perfect" performance many times, no one can be perfect all the time (how many times does a baseball player strike out in a season?)
The easy availability of recorded/edited music has helped create an unrealistic expectation from performers and audiences when presenting or attending performances. In a recording session, the performer gets several attempts to get the best recording, then, in editing, little glitches can be editied out (wrong notes, out of tune entrances, etc) and enhancements can be added such as reverberation to make the instruments or voices sound fuller or richer. After several hours or days, the "perfect" recording is produced.
For a live performance, when you walk out on a stage, you only get one try, and you may not have even had a chance to warm up on the instrument or in the room. There are a lot of variables in that situation, and the chances for a "perfect" performance are slim.
What to do?
One of my favorite college professors would emphasize "set the mood" when I was going to perform. That advice helped me to focus on the meaning of the music, rather than how perfectly I was playing. The middle of the performance is not the time to worry about a technique or even memory.
Accept it will not be perfect! Even Beethoven would tell his students that everyone has "slips of fingers." (And remember, in his day, musicians played from the music, so there was not the issue of memory with which today's soloist must contend. Also remember, the music was NEW! No one knew for sure if you were playing something wrong!) As I tell students, if you are listening to a great speech from an impassioned speaker, the speech will not be ruined if he/she stumbles over a word or two -- it will still be a great speech!
Make the variables part of the music, with a sense of humor. A fire engine goes by in the middle of the performance or someone sneezes? It is now a part of the music, and that is the magic of the live performance! Missed some notes? Quoting my favorite professor again, "make the wrong notes sound right!" He would further explain, "if you will let yourself forget the mistakes, the audience will also forget."
I have heard really fine, world-class performers make some noticeable mistakes, or play sections out of tune. They are still world-class performers, and it was worth my time to hear them. No one can be perfect all the time, and most of us can only achieve moments of perfection. Love the music, and enjoy it as you play and listen, and you can avoid the paralysis of perfectionism.