There are a lot of grey areas here, but my usual fiding is that when students play high, they tend to back off on the amount of air going through the embouchure. This causes the small muscles of the lips to do all the work and tires them quickly as they are not equipped to handle that type of "strain".
As I have stated before, there is a symbiotic relationship between the embouchure and the air - neither one by itself can produce the sound or buzz. But they are NOT equal. The fuel is more important than the motor - the air is more important than the embouchure.
It sounds as though you have over worked the muscles in the embouchure so I agree with the folks that said to take some time off. Next check out this post of mine from a few years back:
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As you go higher, the lips curl in so that you have firm tissue flapping on firm tissue - like a snare drum head. The air stream should be going straight down with faster air under higher pressure as the aperture in the embouchure is smaller and it takes more effort to get the air through.
Read the post noted above and think about it for a while. Get several days rest away from the horn before you dig into it. Let your mind come to grips with the "trick" to playing in the high register. Yes it's just a trick or a gimmick and it will net fast results - but you have to have it clear in your head concerning what you are doing.
Remember - babies and monkeys can buzz their lips. What we do is NOT hard.
Roger
"The music business is a cruel and shallow trench, a long plastic hallway where thieves and pimps run free and good men die like dogs. There's also a negative side." Hunter S Thompson