Ohhh gosh... I know what your dealing with! I used to own a retired racehorse (About 10 years old) & she did so amazing in the round pen but out on the trail or in the feild all she wanted to do was GO!
Here are some things that helped me & my thoroughbred:
I highly suggest that you relax yourself completely while in the saddle. Try not to lean forward while riding her because that is how the jockeys ride & she knows that means run. Sit as deep as you can & really relax yourself. Remember, your horse can feel every move your make. If your tense, she'll be tense. If your calm, she'll be calm. She is looking to you for guidance & it is your job to show her the right way.
I do suggest working on your "whoa" cue. While in the arena, work on walking directly towards a fence & once you approach it, sit deep, say "whoa", & add rein pressure. Do this at all three gaits (Make sure your good at it at the walk before you go to the trot, then canter.).
While your outside the arena & have practiced the things above: Relax yourself to relax your horse. Also, when she goes into the canter without your cue, slow her down to a stop, back up, and continue on in the pace YOU want to go. If she canters again... repeat.
Consistancy. Consistancy. Consistancy.