I'm currently in 11th grade, and I've been playing since 7th grade (total of about four and a half years). I'm in my school's marching band and jazz band.
This past summer, I had an amazing teacher. I practiced a ton using my Yamaha 5C mouthpiece and I got really good; improvements in range, endurance, and tone quality. My teacher eventually moved away. Then I had to literally throw away that mouthpiece for personal reasons. I switched to a Yamaha 7C for a few weeks, then to a GR 65MS for about two months. On that 65MS, my range was cut and my tone was pinched. I quit practicing due to lack of motivation. Honestly I got a little depressed because of my progress being undone.
In November, I got an offbrand 5C Yamaha-imitation. My range is back to normal (D above the staff) but my endurance and tone are suffering.
In terms of tone, my sound is very airy. I've always struggled with that and I don't know how to fix it. I've tried pulling my lips inward, shifting my mouth's position in the mouthpiece and my sound is just awful.
My endurance is terrible. I can hit high notes (above E at top of the staff) for about my first 5 or 10 minutes, then that ability goes away. After that, G and above fall to this really flat screech sound that's impossible to describe. I'd never heard it before. And F and F# make my lips quiver so that it sounds like I'm flutter-tonguing (even though I'm not).
I'm deeply embarassed by my current playing ability, especially because I'm the oldest and lead player, and so many 10th graders are better than me. I'm very frustrated. It's gotten to the point that I dread going to band class because I'm scared of screwing up and being found out and looking like an incompetent fool.
I won't be able to afford a teacher until April. What can I do to help myself?