I am not sure this is a feed issue, really. My new mare, after trying her in various different “herds” is now in a field with horses she really likes and who are nice to her. She is much calmer, more relaxed, and no longer paces in her stall when she does have to stay in the barn. It took four months of experimenting, but she is now a much happier horse (and I am happy that her stall is not so much of a mess every day). Perhaps what your girl needs is a friend, or even several.
It is never the horse's fault