Horses has a very strong feelings,speccially if they have a trauma. I had an andalusian stallion who had the same problems, I spent one year with him... helping him to forget. I started with him as a joung horse, no ridin, lots of grooming, lounging, talking to him, triying to show him I was not his first cruel owner. Its a long way to recover the emotional balance in a horse like that, but, when he trust you, you can start to do more tings, but slowly and lots of patience. Don't try to do same things he suffered in the old times, as spurs, etc... he will remember pain and suffering, and will not collaborate with you.
Good luck , and I hope your horse will be as nice as mine is now!!