The thing is that self-criticism, in the proper amount, is absolutely necessary. When you are alone in your practice room who but yourself is there to say, "that scale was sloppy, better do it again;" or "my arpeggios are uneven, need to practice slower". But self-criticism can easily get out of hand. We also need a deep well of confidence and all that practicing, all that correction of errors, development of tone-color, phrasing and so on, must go towards building self-confidence. You need a lot of confidence to walk on stage and perform!
I am reminded of a very funny New Yorker cartoon from years ago. A small boy, perhaps nine or ten years old, is standing by a piano onstage and saying to the audience, "And now, God help us all, Rachmaninoff 3."
Aristotle really had it right when he pointed out that the right course of action is the middle between extremes. Courage is the right virtue, steering a middle course between cowardice and reckless disregard. Moderation is one of the most important virtues. Excessive self-confidence might mean that you too easily accept sub-standard playing. Excessive self-criticism means that you never have enough confidence.
Let's listen to some confident playing: