Keeping in mind I don't drive my horses:
Do some round pen work with her. Let her trot for a bit and kick her up to a lope to where she wants to slow down. Keep her loping til she's worked up a good sweat. Let her back down to a trot and kick her up to a lope again.
Do this a few times and try to let her come all the way down to a walk.
You want her to remember that walking is a good thing, too.
Put a voice command to slowing down, for example, "Easy girl..."
Just work her in the round pen for a while until you think you've got her to the point where she's listening to you and then try hitching her up. When she's already tired.
Like I said, I don't drive, but when you have a saddle horse that's forgotten their manners, you put them back to square one for a while. I would think a horse is a horse, no matter what their tack is attached to...