I used to have the same problem with one of my old horses. Do a lot of out in the open work to make sure that your horse is okay with it. During your flats you can work on transitions ( halt to trot, canter to halt, canter to walk, etc.) You want to make sure your horse listens to you well so during your warmup work on half halts, your breaks, your go, and your turns. If your horse isn’t listening out on cross country you can circle because horses can’t move fast in a smaller circle.
I suggest circling when out on the field and just doing flat. Think of it as trying to get her bored. Just keep circling in front of a small cross country jump. Once she is calm, turn her into the jump and pop over it as if it’s no big deal. When turning make it a part of your circle. You don’t want to lose the collection and control you have gained or there is no purpose to it. This is what worked with my pony. Hope this makes sense and good luck!