Putting the halti on is not going to be fun... Dont know if you'll be able to get that close to fit it and then put it on.
The goal might be to build up her confidence first by quietly sharing space.
Have you tried quietly sitting on the floor, not looking in the eyes, and doing a trail of treats slowly to you? Just keep the leash and collar by the treats to desensitize their "scaredy-catness". Its got to be a slow, gentle and quiet process...
When working with Jelly the SharPei, it was a very slow process in the beginning. Having a second dog helped, but the initial trust building was just sitting and treating. she got chunky at first!!