Horse that just runs off in canter

"Running" usually comes down to either balance difficulties or the horse not understanding its job and becoming a little stressed or frightened as a result. I would focus on plenty of transitions where the horse takes only a stride or two of canter after the strike-off, then comes back again. Ask for the strike-off over a pole to teach the horse the transition, then work on the canter from the ground through lungeing 😊 Long reining, plus hacking out over rough and varied terrain, will help build strength too. With more strength she will also have the power to balance herself better. Good luck!