Sounds like your mare is barn sour. That's where all her buddies are! That's where all that excellent feed is! Why on earth would she want to leave?
When you're working with her, try to make staying near the barn = work! During this time, it is unpleasant for her to stay too near the barn, as she has to work her patootie off! Transitions, direction changes, the works!
Then ask her to go away from the barn. If she does, reward her. Make the wrong thing unpleasant, and the right thing pleasant. She'll fall into pattern right quickly.