When your not with her it may be difficult to stop her behavior, it may be like weaving or cribbing and she is getting endorphins released when she does it. Is she stalled, does she have access to hay/grass all the time, is she able to socialize with other horses, does she get enough exercise. Her environment and management could very well be creating her behavior.
When your riding her though have you tried redirecting her energy when she starts this behavior? Doing things like serpentines, circles, lateral work, horses can only think about one thing at a time so if your keeping her mind engaged and feet moving she won't have time to worry. There are supplements that you can feed to help her relax, but management of her environment is where I would begin.