I play Mellophone primarily but it functions more similar to Trumpet then Horn regarding Ambochure. Recently, I've started getting proper ambochure for playing tonal wise at least (regarding Horn mostly but to a degree now Mellophone) and now my lips have some actual strength. My issue, however, is that now I'm tending to start thinning my lips or squeezing too much when I go to higher notes.

I try pushing from the diaphragm for air speed instead of lip thinning, but my lips thin anyway almost automatically and I'm not sure how to stop this. Could it be from tired chops - to be fair I've practiced a bit today - or is it something else? Any advice on how to prevent them from thinning on their own even when I don't want them to would be appreciated.​