A lot of barns keep music on for the horses, so it is an old and proven relaxation technique. I’d try it first, because it is cheap and easy. The only other thing I can think of (and it would not be fun) is to sleep outside her stall. Or, you can try SmartCalm, so long as the ingredients would not disqualify her. Without knowing your horse, it is difficult to give really helpful advice.
It is never the horse's fault