The most important thing to consider when playing the horn is "how do I sound?" right? Well, yes, and no. Active listening is what separates a medicore player from a good one. It's the key to having good time, good intonation, good musicality, and the list goes on. However, something that seems to plague young players is their inability to diagnose a physical problem when they can't play something. It's hard for their teachers to do it too. The only person that can feel what you're feeling is you. One of the things I struggled with when I first switched to euphonium was endurance and consistency. I'm not gonna lie, I still struggle with it. Just to a lesser degree. I just couldn't figure out why. I did all the exercises, I played a lot, I played fundamentals (probably not as much as I should have, but I did nonetheless), and I still had the same issues. It took me until about a year ago to realize that it had a lot to do with how tight my corners and my jaw muscles become after playing a lot. I think it's something we all deal with. The old unwritten cure for endurance is just work those muscles out until they're nice and strong. I think there's certainly some truth to that, but within reason and with control.
So, as players we really have to be cognizant of what our face is doing and going through if we're going through a rough patch. We have to be able to recognize the symptoms of certain hidden problems so we can take the necessary steps to correct them before we do more damage.
Now, with that comes the other problem of death by over-analyzation. Try to simplify things. For me, I can attribute 90% of my problems on the horn to my face just being too tight. It causes my lips to protrude too far forward causing a number of issues that I resolve by simply being aware that I need to loosen things up and let my embouchure just relax and sit back. That's it! No more thinking about it. Too often I would have some kind of chop issue and my natural body reaction was to tighten even more. Especially in the middle of a performance. Maybe it was a "here we go again" situation. Now that I've identified the quick and easy solution (which happens to just be the opposite of what my body does naturally) I can get through those situations, preferably without anyone noticing. To them, I still "sound good" and that's all that matters, right!?!
Food for thought.