I’ll start by saying that I’ve not read all the way thru, but:
Think about yourself- if you decide to take up a new sport or yoga or something, you know that it will take a long time for your body and muscle memory to sort things out and be sort of “correct”.
You go from one equine discipline to another, same thing.
You throw in a horse who is also being asked to make a transition and even with perfect training, its hard for him too.
If I were you: I’d find a good trainer ( ie sympathetic to the horse, AND good at rider biomechanics). Cough up the $$ for full training if you can, but with the idea that trainer rides horse maybe 1 time per week so he/she knows what you are feeling or where the horse wiggles, pushes, etc, and what he responds to… You ride the horse the rest of the that week’s training rides. do it for a couple-3 months and see where you are.
GOOD Eyes on the ground are worth every penny.
I had a trainer some years ago that had trained a double-digit # of horses to Grand Prix. But over several years in my early dressage learning curve I figured out that she was actually pretty bad at training the rider. Never told me I was crooked in the saddle, that I have a habit of looking down, that I had no real core strength, that I didn’t know how to ask with my seat, the list goes on. I moved up thru the levels because I (somewhat unknowingly) bought a well trained TB with a heart as big as a house. and a decent go button. His brain functioned like this: “well that sort of maybe feels like the aid for shoulder in so I’ll try and see what happens”; “Oh little give in the reins, must mean extension” etc. Didn’t carry over to new, younger horse…
DONT give up, yes its hard but oh the rewards…