My horse used to try and flatten riders on his back and on the ground by slamming them into walls, charging at them, bucking through fences.. he was very very mentally damaged. He caused a girl to break 3 vertebrae in her spine (she's fine..) and dragged a man along a wall. Everyone was so scared of him, and he even more of them.
But now he feels confident and is more trusting of people, especially if they have cookies ;) , and just the other day I let my friend hop on him for a quick spin and at a tall 17hh.. he was a perfect angel.
It's all about training.. find a trainer or invest the time and energy and RESEARCH (clinics, reading up on things, youtube videos for references..) ways to help boost your horse's confidence and your own.
Even when you think you know your horse, one little exercise here and there and they've completely settled out.