YOu just need to learn to breathe more, the more you get worried and tense up, the worse it will get. You could try using your seat as a bit of leverage, by leaning back slightly in the canter you can slow your horse down, I have to do it whenever Lottie starts rushing. It works for me anyway. Just keep leaning back until you feel him start to slow. You could also look at the way he's making a transition to canter, if he's doing it from a rushed trot then he is already going to be on the forehand and unbalanced, which will make him faster. Try asking for a half halt in the trot before you give the canter aid, that should balance him up a bit :)
Mount up and leave your troubles behind on the ground.