This is a situation where the rider and horse just don't get along. Time to break up the relationship and go their separate ways. They'll be happier with other partners in crime. I don't call that "quitting" or "giving up". Sometimes compatability just doesn't happen.
Reminds me of a horse I bought years ago. At 17 hh and built like a train, he was a tall TB looking quarter horse. Four years old, never been broke - in fact, he was sent to a trainer to be trained but began bucking and the trainer sent him back to the lady I bought him from telling her that he was untrainable. I think that the trainer was 'out-horsed' and tho she had good intentions, she actually reinforced the bucking!
Somehow I managed to put two years on him going slowly - he still bucked dirty from time to time (and I ended up with a broken hip) and got him going pretty good. That horse had scope! He was a superb jumper. But he didn't care for me one bit, and right when I was on the fence trying to decide if he needed to find a new home, that's when he threw me down HARD and my hip gave. Easy decision.
I healed but stayed off of him b/c I didn't want him to continue to think it was ok to buck. I passed up many an offer from some crazy people that really had no clue, to big cowboy types that wanted a horse that could throw a steer. Even tho this horse actually was the first horse to put the 'fear' into me (I was in my late 20's at the time with over 15 yrs of training horses behind me and was never scared of any of them), I still waited and found him the perfect home.
He is now a novice eventer with a lovely family who is using all of his talents to the max. He immediately clicked with their son and from time to time they send me great pics of all of his accomplishments. He has turned into quite an animal over the last several years!
So yes, no matter what your intentions are, how good a trainer you are or how hard you work, sometimes the chemistry just isn't there. Even tho this guy beat me up, I held out - he had 'something' there and I wasn't about to let it go to waste.
So now every winter when I get an achy hip (arthritis) - it reminds of this time I flew thru the air......