This is a challenging horse for you, but I think you can rise to the occassion.
one quick question; how many people in total are riding her? weekly.
I once part leased a mare who was a real goer. she was part arab and fast was her preferred speed. But, when the owner part leased her to two more persons, and occasionally rode her herself, we had 5 different persons riding her somewhat regularly. She finally rebelled, and when she did, she did it in style; rearing, and even striking out and threatening you if you were trying to 'work' her in a round pen. She had just had it! She put her feet down and said NO!
when the owner got her back to one or two riders only, she recoverred her good will.
The thing that might help is to watch her attitude. If you start to notice that she is beginning to 'sull up', which means she is thinking of resisting rather than going, you can sort of shake her out of that mindset by doing something new. Like, reverse directions. Or, come to a complete stop and let her have a long rein. Or, turn her 90 degrees and ask for a rapid depart into a trot , rather suddenly so she doesn't have time to think about doing anything else but going.
the trick is to knock her out of that mindset of resisting, but doing it BEFORE she really gets committed to that way of thinking.
you have to watch her, and notice if she is beginning to pin her ears, or if when you put a leg on, she sort of arches her back, and actually slows down, or she gives her head a bit of a shake. There is a feel that comes on even before these signs, that indicates you are 'losing' your horse into those thoughts.
Grab her back from them by doing something energetic.
I sometimes noisily slap my own thigh, or loudly hiss to break a horse out of this sort of thinking. I am always ready for them to jolt forward, and careful not to jerk them in the mouth with that. It's very improtant that once the hosre moves forward, in freedom, that it feel free to them, so you don't do anything, just let them move forward with energy, if you can ride that ok.