I used to have a thoroughbred who had once had an accident in crossties that ended up with him flipped on his back with his head hanging high in the air. Ever after that we could not tie him because sometimes he would be fine, while others he would freak out like your horse.
Does she ever freak out and pull back when she's not tied or is it only when she's tied? I personally wouldn't suggest tying her up and letting her fight it out like the other two posters suggested, instead I'd work with teaching her to stand still when you drop the lead. This method will take longer, but it is also the safer method and if she is in fact reacting from fear from a previous life event, it would be the kinder rout.
If you do end up tying her, however, I would suggest tying a rope around her barrel (with padding underneath so that it doesn't rub and make sure it's not a knot that can be tightened by pulling back) with the long end running between her front legs and under the nose band of the halter. Then tie both the lead rope and the rope from her body around the tree or whatever unbreakable thing you are using. Let her fight until she will stand completely relaxed without bracing again the belly rope.
I had to do this with my filly when she was around a year old and decided she no longer wanted to stand tied. After one 20 minute or so session, we never had to repeat it. Of course she tends to be a fast learner.