I've been playing trumpet for close to 10 years now, and lately I've been working a lot on range, as I've been moved up to play lead. Last year was the first year I really worked on range, and my range soared within a month of range work. Starting near the end of last year, my range suffered mercilessly (went from a strong double C to a weak high Eb). My teacher and friends attributed this to playing too much in the upper register and damaging my muscles. Over the summer I had several periods of range building, followed quickly by range drops again. It's now been a full year since I started working on range, and I can estimate it's been about 10 or 12 times now where I've built my range like crazy and then it drops just as suddenly as it came.
I saw, from doing research, that my embouchure might me the cause of all of this. When I play in the upper register, do lip trills, or just get tired, my corners involuntarily form a smile embouchure, the embouchure that I've heard so many bad things about. This is completely out of my control, however, I've tried forming a pucker embouchure and I can't get a sound out, and as hard as I try not to smile when I go high it's simply impossible.
Could this be the source of my awful range problems throughout the past year? If so, how can I fix it? Again, the smile forming is completely involuntary.