Is the halti leash the halti head collar? If so I would say not to use that... that would probably be worse for her.

You have to slowly get her used to it. Start off with a very light, thin leash or even a small piece of rope, something very lightweight. Leave it on just for a few minutes multiple times a day. At the same time you can teach her a leave-it command, tell her leave it in a firm voice & gently tug the leash away. BUT at the same time you want the leash to be a happy object so don't yell too loud or tug too hard. When she is not bugging at the leash make sure to PRAISE PRAISE PRAISE!

Slowly start to lengthen the lead & start to introduce a heavier lead if need be.