Don't forget this one too.
I have a different approach for you. Scott E. mentioned that performance anxiety does strange things to the body. It is the same as our fight or flight response. When one is attacked, the body tenses the torso for strength and that is done by closing things off at the throat and compressing the muscles. Try inhaling and say "uh!" as if someone just surprised you to get the idea. When we put so much of ourselves into making music, we feel stressed. We want to sound good and do well. Under these circumstances, the body reacts the same.
My thought is that although you may breathe shallow, (there have been some good posts to help you with this, particularly the reference to the Arnold Jacobs site at windsongpress.com) you may just be compressed. Pressurized CO2 in our lungs makes us feel suffocated in the same way as when we have no air.
The compression happens in between the inhalation and exhalation. This is the moment when our mind says "PLEASE DON"T $%^$%*$%^ UP!!!!" In order to circumvent this natural hesitation, we need to build a new habit. Arnold Jacobs used the words "oh" for inhalation and "toh" for exhalation with no hesitation between. If you practice using these words while you practice breathing in and out you will get more air in and it will come out relaxed and easy.
Do this excersise without the horn and then bring the horn to your lips while continuing this cycle. Do it several times and then suddenly allow your lips to create a sound. Make sure that you hear those actual sounds in your head and continue to breath in up the the moment you release the "toh" into the horn.
Try to incorporate that breathing in your warmup and daily practice and it will eventually work it's way into performance.
Best of luck!
Eric Berlin Majestic Brass Quintet
Principal Trumpet-Albany Symphony
Majestic Brass Quintet