So, what we have here is a confidence problem. You're uptight about your ability to play the music and it's causing you to choke.
What you need are some strategies to build confidence for when you are playing 'real' material (etudes, band music, tunes, etc.) BTW, a lot of these will help with sight reading too:
- before attempting to play a piece, analyze it. Look ahead for potential trouble spots -- meter changes, key changes, tricky passages, long passages with no breath, ds and codas, hard articulations, etc. If need be, spend a little time on these before attempting the whole piece. NEVER LET ANYTHING LIKE THIS SURPRISE YOU!!!
-before attempting to play a piece, understand the structure of the piece -- what is the form? Understanding the form helps because most forms have repeated sections. If you can play the section once, you can play it more than once....if it doesn't catch you offguard.
-practice the piece in sections. Instead of trying to tackle the whole thing at once, take the first A section or the bridge and work on them separately.
-be very aware of your 'condition' when you start playing -- the NANOSECOND that you become aware of tightening up, stop, think, what's going on? Why did you choke? Do you feel tense, breathing correctly, air column working, etc?
-by stopping and thinking about what's' happening, you may find that certain material is shutting you down - either you can't play it, or you THINK you can't play it (lack of confidence). I.E. a difficult fingering, tonguing, etc. IF you can isolate the offending material, then go find exercises in other method books to work on the problem until you are confident.
Hope this helps,