Could be paralysis by analysis. Take some time off, get your head straight and try to come back to the reason you play in the first place. Why did you start playing as a child - what excited you about music? Chances are it didn't have anything to do with auditions or choking. People make mistakes - we are human beings. Live music is thrilling precisely because of the possibility of change and more than likely no one even knows or remembers whatever mistakes you made. We all remember our greatest chokes - but I can't seem to recall anyone else's. You have to treat mistakes as the gift that they are - a moment in time that are important to YOU and then vanish for everyone else. And there will be more - forgive yourself and move on.
And as with anything - when you get back to the horn it's long tones, long tones, long tones. Spend more time on long tones than you want - and then spend some more. Take a big breath and make a beautiful sound. Let everything else vanish with the mistakes.
Adams E3 0.6 with SS Bell
K&G 3.5D
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Founder and Solo Euphonium
San Francisco Brass Band