make sure it not a pain issue or anything but if it’s not a pain or physical issue I would smack her with a crop or dressage whip on her butt and make her canter around
Make sure it's not a physical or pain issue first, and if it isn't, get a professional to help with the problem. A young horse with behaviors like that can be dangerous if not addressed early.
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.
I am getting into the training business. Let me know where you are, I may be able to come out.
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.
Try to know why she's bucking