I trained a wild mustang & now she's a jumper so I know the feeling. The best thing to do is get respect on the ground doing in hand work first then ride. Because if you have respect t on the ground, you'll have respect t in the saddle. You shouldn't hop on until you have confidence your horse won't have such a big attitude. Good luck!
I've had this problem with a lot of horses before I learned to be durable in the saddle. You need to find a person that is almost impossible to overthrow. After many unsuccessful attempts she should calm down and realize that you or someone else will not hurt her.And do not forget to reward good things.
Figure out if there is a problem, like ill fitting saddle, bad teeth, something out of place in her back, etc. Then if all physical problems can be ruled out, try some simple groundwork with her. Maybe "restart" her and see if you run into any problems.