Time , patience , time , patience - and understanding
Having had a couple myself I cannot stress how much you need to learn to read your horse . The welsh A was so badly treated that after 8 years I am still the only one that she trusts enough to catch in the field , and my arab/welsh/tb cross would attack anyone but me that went near her when she was eating , neither case is the horses fault - and neither is told off for such behavoir , every one who visits is told the rules regarding these two and if they break the rules then it's thier fault - not the horses.
You must gain the horses trust - I had to spend many hours with the second case just sitting in the field with the horse on the end of a rope doing nothing at all except getting the horse used to me - and getting her to know that I was not going to do anything to her.
Stability and routine is the only thing that worked with the welsh A.
A Horse isn't born bad - It is just made that way by US