Per the recommendation of a very experienced, generous woman who boards at the same barn that Jazz and I do, I was borrowing one of her ultra-wide saddles, and her Parelli Theraflex pad, which I thought were very logical, and seemed to fit him well. Jazz was honestly pretty awful yesterday, when I rode him, so today I tried something: I put his old saddle (which I'm just borrowing from Jazz's owner (he's a lease) until I can finally find a saddle in my price range). Anyway, I put that saddle back on him and voila! He actually offered canter transitions willingly, finally got back to working on the bit, etc. And when I let him stretch down to a long and low trot, oh my god, he was fabulous! He dropped his nose practically to the ground, his back raised noticeably, and he moved with this wonderful springy trot, full of expression! It was crazy.
The craziest thing of all, is that his old saddle doesn't fit him anywhere near as nicely. The back of the saddle has much sharper pressure points than the ones on the saddle that I was borrowing. And yet, he apparently hated that other saddle. I just don't get it. It's bizarre. I'm very, very pleased that it was just the saddle though, and not me riding terribly, and not him just being bratty. I'm so glad to have my happy, willing horse back.