Having a horse that is able to be ground tied is invaluable. Try to teach her that.
Another thing to do is try to see what it is that sets her off. Take the lead rope (while she isn't tied) and pull back, kind of simulate her hitting the end of the lead if she were tied, and she what she does. It's most likely that she gets scared when she feels that resistant pressure on her poll. It doesn't let up, and it makes her feel trapped (a horse's worst fear as a prey animal). She needs to respond to the pressure by moving forward, so you'll have to teach her that.