Greg, John and Wildflower (though in no particular order): you have all been so kind and generous in giving your time to a stranger.
I’ve been feeling my emotions very close to the surface as I’ve been writing about this (some mix of shame and rage/frustration, I guess) and that seems a pretty clear sign that I’m not approaching my playing with a particularly helpful mindset. Wildflower’s mention of possible emotional and psychological components getting in the way certainly rings true as a possible component of my own situation, as does John’s wondering aloud about whether I might be playing with lips insufficiently relaxed.
I’m going to take up Wildflower’s and Greg’s suggestion, and arrange an online session with Greg.
I must repeat, though: you guys are wonderful, and thank you!