By George, I think I've got it!

I don't want to jinx myself or anything, but a few days ago, I noticed the sitting trot had suddenly gotten much, much easier.
Tonight, both the sitting trot and posting without stirrups were ... a cakewalk. I didn't even have to do the baby slow western trot.
My back is moving different, which lets my hips do the nordic track, but I'm not doing it on purpose. It feels great. I hope I can keep doing it.