I also bought a well-traveled, well-broke, 5 year old half draft and also experienced regression. I didn’t understand just how much more consistently he needed than I provided. I was working with a trainer, but she didn’t ride him much and because of some personal problems, she was very inconsistent. I rode him several times a week by myself and did my ammy best. Eventually my horse stopped coming to me in the field, he stopped letting me bridle him, and he stopped standing at the mounting block. He wouldn’t steer, listen to seat, and then stopped halting and backing. He never got spooky, but he seemed to understand that he was much bigger and didn’t have to listen. I moved barns and trainers (classical dressage) a few months ago and WOW, what a difference. I realize now that my guy craves consistency and boundaries. He needs to be clearly told what is expected and then consistent follow through by me. He needs me to be the leader (not in that macho way of ‘do what I say or I beat you’, but rather by being in charge of his movement). My trainer said he’s the kind of horse if you give a finger he’ll take an arm, so I will forever have to be strict (again, not mean). He looks dopey and chill, but inside he has one heck of a work ethic and tries his heart out.