Another thing I have heard is that when a horse has "a purpose" to go somewhere, he will often go there with vigor. Like chasing cows and such. What if you played chase with somone, perhaps on the ground (you being mounted and them playing the chasee). Or even started some sort of polo, if she isn't too shy of things being swung around and over her.
I was looking at this website on the Canadian trainer, Jonathon Fields (spell?) and he said he had this horse named Quincy who was THE most difficult horse he had ever had and he thought he would never get anywhere with him. When in a brainstorm he started doing "charro" type stuff, riding iwth a long stick and doing manuevers, that Quincy just took off, as if HE thought it was HIS job. From then on when they had a job to do, Q was a partner, instead of a slave.
So, think about doig something like chasing a dog or a person or a person dragging a tarp or kicking a ball, or . . . But be so focussed on gettting to that destination quickly and get her invested in the chase.