The copper rollers MIGHT help her with stopping but I think iridehorses had amazing advice. They will keep her mouth moist and might encourage her to soften. A horse with a tense jaw or lips won't feel it as readily. Imagine this, if someone is going to punch your arm, and you know it, you brace, making it hurt less. If you aren't expecting it, and they wail on you, it stings almost twice a bad.
There for, your arm is more sensitive when it is relaxed. The copper rollers, can help a a horse relax its jaw, making a more sensitive mouth.