Nick, remember when you had played trumpet for a year or two and you could play up to the G on top of the staff? Everything else above that was hard. You almost always cracked the "A"! Every once in a while you could squeeze a high C out and that made you very proud! What did you change to get above that G? Most likely the pressure that you used on the mouthpiece was reduced, probably a bigger more relaxed breath, maybe a more focussed embouchure. In every case, better controlled with LESS FORCE. An octave up, we have the same problem. The lips do not just stop working at high G. We generally apply FORCE first to get there and that squeezes off the airflow or puts pressure on the upper lip and THAT stops the vibration. When we back off on the force, the lips are at least free to vibrate higher than high G and then we need to make sure that the "airflow" is sufficient to play the note. A useful Double C fairly loud requires 3 atmospheres of air pressure. That is enough to pump up the tires on a racing bicycle or blow up a hot water bottle like a baloon (I do not recommend doing either - it is bad for the cheek muscles!). The airflow and reduced pressure issues are things that I do not talk about on the internet because the actual procedure depends on the player. Here we have body use, breathing, embouchure, tonguing, overbite issues that I have to physically see to comment. No trade secrets, just too individual! There are also enough players that seem to do everything "correctly" but don't get that last high octave. I can only speculate what is wrong there.