That would, to me, fall under manners. Maybe she needs a refresher course in those. A horse who doesn’t do what you asked is not much fun (I have two of them). Does she longe? Maybe you could do that for anywhere from 10 to 20 minutes before riding? Another thing, if you have enough space to do it, would be to make her turn in spirals, or turn in a direction in which she doesn’t want to go? Just be careful there is sufficient space that you and she don’t fall down. How does she carry herself while having all this fun? Collected? Strung out all over the place? Mine get strung out, so we work a LOT on collection, and if they don’t behave well at the walk and trot, we do not canter. They have both learned that if I say “halt”, I mean it, so that has helped. I would suggest that you find someone (a really experienced someone) who is qualified to help you, even if you have to trailer her miles and miles for lessons. Somewhere in the multitude of pages of my post about explaining that a show ring is not a racetrack, G&S gave a very good description of how to stop a horse, using all your muscles. Perhaps that would help you as well. It worked for me.
It is never the horse's fault