Rain
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But yeah, I also have some of those "what the heck am I doing" moments...
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The best ever on these moments was with Andres Segovia in Madison. He was doing a piece, and broke a string and never wavered, shook, or anything else ... he continued ... and at the end, he goes "so sorry about that ... did it sound alright?" ... and promptly got a standing ovation!
It is, one of the hard things that you try to teach an actor when rehearsing and working on something. Those moments can break the concentration, and the trick is not to break the concentration at all.
Think about your performances with an instrument ... !!! You have to stay focused ... and if you have "doubts", then you should not be there! That is usually, the difference between a winner and the rest!