I also agree with Peyton get a buddy to go with her, but be sure she doesn't get buddy sour.
I agree with Peyton.
Is she turned out with mares or geldings? How many horses is she turned out with? Usually horses all get along best in even numbers. When there's 3 or 5, one gets left out, picked on, or always chased away.
Try a buddy system. Rather than keeping her in a herd take one of the other horses who is low on the totem pole and put them together separate from a large group.