Any advice for an instrument switch? (alto to mello)

Im a sophomore going into my third season this upcoming band camp, and im going to be switching from my usual marching instrument, alto sax, to mellophone. i chose that because its what my director suggested and because ive always dreamed of being in DCI and I i needed to change my instrument if i was going to do that, and im doing it now because i want as much experience as possible. i want as much as i can because im aiming for one of the top 6 corps (of course not my first season) anyways that said, ive been practicing mello for a while now, and im decent-ish, and ive been taking private lessons and and all, but my chops run out after 45 minutes and i cant reach some notes in the higher octaves. my tone is also fairly muddy and really bright, do any vet brass people have advice on how to work on these issues? that and any other info on the subject would be greatly appreciated, thanks!