Backing her into the barn is what I would do. We had a horse who narrowly escaped a barn fire when she was young. afterwards she was terrified of going near a barn or stall for months. What finally worked was A LOT of positive reinforcement. And food, lots of food.
Back the horse until she won't go any further, then turn her to face the barn and reward her. don't ask her to walk forward, just turn her around again and back up another step or two. turn her to face the barn, reward her, and repeat.
If the horse is comfortable and responsive when lunging you can try to get her to lunge at a walk in front of the barn if there is room. Every few laps you should take a step towards the barn and force her circle in that direction. If the ground is dirt or grass, then keep her focused on you by changing between a trot and walk regularly.
If you can approach the barn from angles besides directly towards the door, you should try this as well