Perhaps she is reacting to something other than being alone in the barn. Maybe there is a smell in the barn that she is reacting to or she has a strong memory of something unpleasant happening to her in another barn. Try using essential oils on your hands, especially lavendar which is calming, and let her smell your hands before even approaching the barn. That might help to relax her. Or use accupressure points that are associated with calming a horse. Just some ideas.