How about teeth? Did the vet look at her teeth?
If teeth are fine, then I would work her over ground poles. You'll need to fiddle with the spacing to get them just right for her stride. Generally, you want them 3-4' apart for a regular working trot. Working her over poles will force her to regular her speed and watch her feet. They will also help improve her balance.
Leave 4-6 poles set up in the arena. When she rushes, turn her in and take her over the poles. Keep working her over them, back and forth, until she stays at her working trot when not in the poles, then go back to the full arena.
Another thing I would work on is lots and lots and lots of transitions. Walk/trot, trot/canter, walk/halt, and transitions within the gait. Work from slower trot to working trot, back down to slower trot. As soon as she speeds up, take her down to a walk or up to a canter, or down to her slow trot.