I was told by my instructor 34 years ago, that if I did not improve my embouchure and use mostly upper lip, then I would not be able to advance very far. I was trying to cram my bigger than normal lips into a 5c mpc. I quit playing in 10th grade, as I seemed to have plateaued, and I could not play in the upper register for an extended period of time.
Fast forward to the here and now, and I finally took his advice, and upgraded to a Giardinelli 1c, a mpc. that my lips actually fit into. Eureeka! I can actually fit my lips into this thing,play with mostly upper lip, and play w/out giving out prematurely. I can still hit the high notes, although that did not come back right away.
It might not work for everyone, but it seems to work for me! I wish I could have afforded a better Mpc. when I was in High school!!