This is an essential, and frustrating, part of a musician’s life: to never believe that your last performance was quite good enough.
I was talking to students about this yesterday; as we improve, we know we can improve more. Perfection seems to be that unattainable goal. We do something well and think, “It could be better!” It’s frustrating sometimes, and can cause a lot of distress, but if we look at it from a different perspective it can be challenging and even invigorating.
I read the quote here. Of course reading about the oboist that Jon Kimura Parker heard I’m also thinking, “He’s probably testing a reed, and has to see how it will handle the solo!”