I second the Blocker Tie Rings. They're relatively inexpensive, and even if it doesn't help, it can't make the situation any worse if they're used correctly (i.e., use a 20-ish foot rope, attach the ring at eye height, and use the appropriate "setting" for your horse).
The key is in the fact that the ring allows the horse to move her feet in response to her claustrophobia/fear/whatever-cause-of-pulling, without her getting the release of halter pressure until she stops pulling, as opposed to breaking something and getting a release/reward for the pull. The tie rings also allow for an instant release of pressure when the horse relaxes, as opposed to a bungee or an inner-tube that keeps pressure on until the horse comes all the way back up to the post, disconnecting the release from the choice to stop pulling.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown