Here's my take: Lots and lots of slow work at the rate of 90% walking with the balance trotting time - no cantering at this point. As soon as she gets "excited" in the trot it's back to the walk until she quiets down and relaxes. Repeat and repeat this; ask for the downward transition in a very consistent sequence of weight/legs/reins. I've had a couple of OTTB. The key to success with them I found was always going back to walk to get them settled mentally - they`re were hot blooded and they learned best when there was great calmness around them.
You may well find a bitless bridle is helpful in your situation. That is how I rode one of mine and she went extremely well in it. Also remember, circles are your friend - if she`s getting overly excited in the faster paces, you`re better to start circling her and asking for the downward transition in the circle than start a pulling contest when going large.
Best of luck.