Somehow she has gotten a negative association with the halter. Are you the only one that handles her? Is it possible someone popped her w/a halter recently? I think first you need to reintroduce it to her in a non threatening way. Let her smell it, lip it, rub it all over her when she lets you. Give her a treat out of the same hand you're holding the halter in. Rub it on her while you're scratching her favorite itchy spot. You need to reteach her that the halter means only good things. Do you only go get her when it's time for her to work? Maybe go get her only to groom and pet her and let her back out.