When she runs make her stop get off than desensitize her to the object
I agree with Ines. Desensitizing is a good thing to do. You cannot be weak as a handler. Approach it like nothing ever happened. Go near the object, but far enough she won't run, and move her feet. Lunge, back her, pivot, yield hindquarters, side-pass. Keep her mind thinking about her task and not on the incident. Then move closer. Continue this until you are right next to the object and let her stop and praise lots. Your ultimate goal should be for her to touch her nose on it. This will take time. Work with her everyday. Good luck!
Desensitize as much as possible. Try four days a week. Be very confident approaching the situation or the object of her fear. She will depend on you for guidance. Have her approach the object or situation herself, give her time. If it helps you can use another confident horse to use as an example for your horse