I have the same problem. The discipline to learn to relax is really helping me. Remember mouthpiece pressure is not your friend. Keeping a tight lip and holding it there is not the way to go. Your lips need to buzz freely. It's easier said than done. Believe me. I just shot my chops tightly puckering to belt out the upper register.
Now I'm on break for the next 20 minutes :)
I think the best way to imagine the mechanics is that your lips are the motor buzzing...lumpity lumpity lump lump...like a wild v8 camshaft. And your mouth and air passage are the cylinder walls. Do not rely on mouthpiece pressure to aid in the compression of your engine. You will prematurely wear the walls of embouchure and your foundation will crumble.
Good luck...Yes I just hit super high tea kettle sounds...Now I just did that because I did exactly what I told you not to do...Relax...relax...Keep your engine in tune.