So what? Let her buck. Why is that a problem? Let her buck but keep insisting that she move around. She has her opinion and she is saying it, but you just keep on asking what you are asking. Stay calm, don't meet her buck with anything more than calm insistance that she keep going. She will tire of bucking and will be swayed by your calm , confident aura and she will start coming to you mentally.
If she is coming AT you, and I don't mean just turning alittle toward you when she is bucking (I would pretty much just blow that off and ignore it, it's not serious threat gesture) But if she is really cutting in close to you when she goes around, leaning her shoulder in at you or multiple times passing you and kicking out at you as she passes, then you need to DRIVE her and get her to realize that you are in charge.
As soon as she surges forward, let her go, don't add energy for a bit, then try again to ask for the gait that you want. Calm Clear And Consistent.