I had the same problem with the horse I have now. He was a dream when I first bought him, but after a while he started taking off towards the fence and would do a sliding stop right before the fence and rear up all the time. I had no idea what to do so I called a riding instructer and asked her what to do, all I had to do was :
Pull back on the rains pretty hard and kick as hard as you can. Sounds pretty brutal, but it actually works. I'd hold on if I were you, because you dont know exactly how she's going to react. If she doesn't really respond, I would get a pair of "humane spurs" that look like
this if you dont use spurs already... good luck, I hope you are able to make a decision!