On April 24, 1839, a 21-year-old Henry David Thoreau remarked in his journal:
Why should we concern ourselves with what has happened to us, and the unaccountable fickleness of events, and not rather with how we have happened to the universe, and it has demeaned itself in consequence? Let us record in each case the judgment we have awarded the circumstances.
The breadth of application for this sort of remark toward a more positive interpretation of existence is wide, but as a musician, I can't help but read it as a comment on performance anxiety. The more we can become casual observers of experience (rather than forcing experience upon casual observers), the less affected we become before, during, and after performance. I think this mindset stands in contrast to the one commonly experienced by performers, in which the most severe of critics occupies our headspace for the supermajority of our time, not letting a single mistake go by in practice. Attempting to replace this firmly-entrenched mindset when it comes to performance time can be a real challenge, and failed attempts have resulted in botched concerts and lowered self-esteem worldwide. Thoreau advocates instead for a probing existence at all times, free of self-judgment, one in which we value ourselves more than the circumstances we find ourselves in. Practicing, then, is the joyous solving of problems and gathering of positive experience, and performance the joyous, selfless experiencing of those solved problems.
And although you've heard it before: Think as though you are, and you will become
As a supplement, here is Muhammad Ali: