Help with High notes

I should clarify: by “high notes” I mean any note that is above a space E on the staff. I have been playing trumpet for a decent amount of time and play for my high school band. I currently use a Bach 7C mouthpiece as I’ve been told I should stick with it until I can comfortably play up to the G at the top of the staff. On a good day, I can play up to an A above the staff without too much trouble, at most dynamics. But on other days, I can only play up to about a staff E that I was just mentioning, and if I try to play above that, I can only hit about 2 or 3 of those notes in a row before my lips instantly get tired and I physically cannot play the notes anymore. Trumpet in general just becomes uncomfortable.

I find sometimes I end up resorting to pushing the trumpet against my mouth during these days, not because I think that’s how I should be playing but it just seems to happen naturally and messes up my lips

I’ve heard advice like “tighten corners while keeping lips soft” and “blow faster air”, but that hasn’t seemed to help me and either makes the notes really multiphonic-y or makes it harder to actually play the notes with air rather than easier.

How can I have a more consistent embouchure across multiple days, and more importantly how can I make playing those “high notes” (E5-A5+?) easier? Any advice would be greatly appreciated. Thanks :)