Use leg for forward, when she gets faster press down with BOTH stirrups while KEEPING calves on her sides (i.e. Keep her in front of your leg) then ask for a halt. Get her so she's:
1.) Always in front of your leg, even if she's running. This is the #1 thying you need to do
2.) Next teach her to halt with her hind legs directly under her body (by keeping your legs on when asking for halt (boot her if her hinds legs are out behind when you ask for the halt). When you ask for halt press down in both stirrups.
Point is to teach her to halt by stepping underneath her self with hind legs not pulling back. When she feels you press down with Both stirrups stop YOUR body from following her movement then close fingers. Last resort (if she hasn't stopped yet) is pull back.
Eventually you want to eliminate the "pull back".
So when you get all that work (i.e. No pull back but horese halts with BOTH hind legs directly underneath her hips, and hopefully "square") then the next step is to introduce the halt halt. This is what you can use to slow her down.
Half halt is asking for a halt and as she steps under her body to perform the halt you STOP pressing down on both stirrups and start following with your body. When done correctly horse shift weight back onto hind end and slows down.
I like to alternate the HH at the walk with alternately pressing down in ONE stirrup (at a time) in the same cadence (of the walk) that I want the horse to maintain - horse will start to follow that cadence (eventually) - add in halt halts and/or full halts as needed to slow horse down.
Note that in all this you are not in the horses mouth (except in the very beginning when you asked horse to halt and she didn't understand). Horse will soon learn to pay attention to more subtle commends with your body so you don't have to get into her mouth. So her being ultr-sensitive in the mouth will help (at least at first).
I'd also suggest getting an equine dentist (or vet) to look at her mouth and "float" her teeth/pull her wolf teeth if that's what she needs done. That will also help mouth issues.