I've been playing for around 6 years now, I'm a senior in high school. I'm playing Petite Piece Concertante and one other selection for a college audition. I can get through the "lyrical" opening, but then I feel like I'm ready to "check out" as far as my chops go. That's not a good thing, considering I still have the "march" section and another selection to go after that. In case you're curious, I'm playing on a Jupiter XO Tribune (about 4 years old) with a Schilke 13C4 mouthpiece and I practice for at least an hour and a half a day. Any suggestions on what may be the culprit of my endurance problems?