I can't give you much riding advice, but basically the way horses learn is the same as in groundwork.
I don't know if you are familiar with clicker training. For me, clicker training has proven to be most effective - and fun
- to level out some communication glitches between horse and human.
The point is that you have a marker signal (click, voice) to mark the behaviour you want to reinforce. The horse is conditioned to associate the marker with the reward it gets soon after (every time!). That way, I can reward the slightest modulation of energy, like a slowing down for example in response to my cues. As soon as the horse gets the idea of what I'm aiming at, the progress from there is much easier and faster.
A golden rule in clicker training is, if an exercise doesn't work like intended, break it down into smaller steps and reward those. Then piece them together to get the bigger picture again.
As I can read from your story, you already have a wonderful communication going on with your horse. And I think you are just inches away from what you want to achieve