When I started to struggle with this same issue, Donald Redux told me something:
Assuming that the rider is technically perfect and the horse is adequately conditioned for the task in question, there are 2 important ways to "overcome" resistance and/or disobedience and those are 1. escalation or 2. persistence.
Escalation is what I think you are currently doing and most riders do it. If the horse does not respond then ask louder.
As if a deaf person would understand us if we just shouted. However, as was clearly pointed out to me, it is not the "asking" which trains a horse, it is the "release" which trains a horse. This was VERY hard for me to learn. If I ask something, I need to ask it, stop asking, and WAIT for an answer.
Difficult lesson for me to learn, but I'm starting to get it
much to Freckle's obvious relief.
Persistence then grows out of learning this "release is more important than ask" idea. So now, if I ask and get no response or a different response than what I hoped for, I will tell Freckles "Thank you. That's very nice but it is not what I wanted. Can we try again?" and I will ask again in a slightly different way. And again. And again. When we reach a point where I am certain he understands my question, ALSO that he knows what response it is that I would like, only then might I consider an escalation if we are in a potentially dangerous situation. Oh yes, and if I get a small response in the direction I want? I reward. A lot. Often I will deliberately stop after the first small "in the correct direction" response and leave to sleep on it. He has usually made a lot of progress overnight. If I stop on an aprroximation of what I'm trying for? Often he will show me a vastly improved behaviour the next day.
So if I ask "turn right" and he says "No, turn left" I will tell him "that was a nice and balanced left turn but I wanted to go THAT way not this way so please can we go back to the top and try again?" I will not click and treat him for the "wrong" turn, but I will praise him for "doing something" because I do not want him to become "afraid to guess" or "afraid to say the rider is wrong"
I will then take him back to the exact place where I asked for the "turn left" and I will ask again but this time I will try even harder to make my request absolutely clear. If he still turns the other way I will then take that turn and make it a small circle until we end up moving towards where I wanted to go. When we get there I will show him what I wanted to do over here. Then I will go back to the beginning and try again. I find that he does "obey" after he understands where and why.
I don't know if any of this is at all helpful, but just know that most of us have no trouble at all doing AND groundwork, and a LOT of trouble doing AND riding.
You are not alone. The answers and solutions we have each found work for us and our horses. You will need an individual solution for yourself and your horse. You will find it. Just persevere, even when you think it will never improve. It will. It takes time and composure from you and your horse before things can start to become clear.