I might add -- long tones - in the staff -- allow your muscles to build up without stressing them --- and you should rest as much as you play during this long process. (again think in terms of weeks and months to evaluate progress -- and don't think in days -- like tomorrow you will be great ))
play 5 minute - then rest 5 -- play 10 -- rest 10 ---- that allows the blood to flow and replenish the muscles.
also -- do a heavy day (lots of long tones) --- and follow that by a light day (not many long tones, and maybe cut the practice back a bit also ---- HEAVY DAYS, back to back, and always trying to play HIGH NOTES will actually inhibit your muscles to build in your face --- those muscles need to recover -- and rebuild -- and it takes a day or so at your younge age --- much longer for me at 46.
***another point is high notes -- take low pressure of the horn to the lips -- just enough to seal the mpc --- anything more will crush the lips, stop the blood flow, and basically make your sound suck -- those lips (in the center where the aperture is) must be free to vibrate -- that is what makes the sound -- crushing them with pressure equals bad sound, and less endurance***