I understand this kind of frustration. I have quite the same problem. I could hit some note like high C or above, but never really did it like a knack. Meaning : that was somehow lucky or not understanding the process, or with pure force using arpaggio’s like in Gordon’s approach.
I have put away my trumpet since a few months… But in the end, I guess, as long as your sound is improving and some things are getting easier, that’s for the best ! And as many trumpeters who want to have a greater range, including myself, we should be happy that something is improving, and focus on that. If you got worse, that would be different. But if you improve…. great !
That means someday you’ll get a better range too. But, maybe, I said with a lot of precaution, some bigger work should be done where things begin to get harder.
That said, on my side, I think I didn’t put enough energy on getting higher, and by energy I mean time, thinking, and process. Not pure force 🙂
But creating a good and solid foundation is good too.