I would lunge her on a line mostly at the trot with side reins (Not to tight)
When riding her I would work on a long rein and again mostly at the trot. If she rushes and starts to canter, I would guide her onto a circle, sit heavy in the saddle to make it harder work for her and when she comes back to the trot, start rising and work large.
If she is lazy then I would push her forward.
Racehorses are use to a constant pull on the reins and when they do not get it they will panic and go faster. Use your seat and body to balance her, you can regulate the trot by rising slower and lower to slow down and higher to lengthen the stride.