TAF... here is something for you to at least try out. When the toilet paper is at the end of the roll you have a nice, open cardboard tube... right? (OK, if you don't want to think of toilet paper.. it's Christmas.... use a short length of the tube that the wrapping paper comes on) Here's what you do.... stick one end of that tube into your mouth, wrap your lips around it and breath deeply, in and out. Do this for a few cycles. Now ask yourself.... do you breath like that when you play trumpet? Well, you SHOULD. Another mental image for you.... early in the morning when you first wake up your body is entirely relaxed. When you speak or sing you do so in a looowww, Johnny Cash-like voice. Why? Your throat is nice and relaxed. Play like that. Do your playing exercises and etudes like that... Remember... throat open, relaxed. Breath from your GUT, not your "lungs". That means let your belly expand down below your belt rather than having your shoulders rise. One online article that REALLY helped me (I'm nowhere near your age.... thankfully) is this one: http://www.markvancleave.com/mvcresintonation.html . It might be the best thing that guy ever wrote. Don't take his words "literally" but rather THINK about what he's saying. Get your body in tune with the horn and the horn IN TUNE with the band and the work load drops right off and your endurance increases dramatically.