I will be a senior in high school this fall and have been serious about the trumpet for the past several years. I am a lead player as well as a jazz soloist, a tough combination I've been pursuing for quite a long time, and am working classically as well. I started (yes I know, only started) doing long tones regularly about 3 months ago because my tone was not where I wanted it to be and I was having trouble sustaining pitches for a long time. The problems I ran into when I started were straightforward; my breath ran out after what seemed to me as an unusually short time (about 15 seconds on each note, 18 if I was lucky), and my tone would quiver quite a lot, especially later on in the long tones session, even though I had good control over my intonation.
The problem is I never stopped running into these problems. My tone has certainly improved since I started doing long tones, no question about it, but my lungs give out after no more than 18-20 seconds on each note and the quivering has not gone away or even lessened at all. Is this a normal problem to persist? Clearly I am getting something out of long tones, but my breath supply has stayed mediocre at best (in my opinion) and my tone still quivers. I make sure to be as relaxed as possible when breathing and playing, take as full breaths as possible, and rest for exactly how long I played between each note. What else could I be doing wrong?