Originally Posted by rowuk

It sounds to me like you are using force to get the notes. When you put the horn down, the lips swell (like anything else on your body after it gets beat up) and gone is your range.

Try practicing for a month up high at volumes no louder than mp - piano would even be better. Clarke etudes are very good for this. If that doesn't work, you are not ready!

Hitting high notes is USELESS if you do not provide musical content, and that reliably.