The answer is in your mental rehearsal of the music. If you repeatedly imagine the sound of great music, then try it on the trombone, eventually your body learns by trial and error what it has to do to reproduce that sound. They suffer from the universal human fear of looking stupid. Better to get it with a bad sound than miss because of the fear of sounding bad. A wrong note is better than no note at all.
People tend to get stuck on plateaus.
It is important to retain our faith in that wonderful learning machine, the human body. Inevitably, with calm, patient persistence it will get by that wall. For example, I had a problem that bothered me for almost twenty years the Valsalva Maneuver. Then, at age 37, I fixed it. Ensemble Playing Most of the time we play in ensembles.