How to slow down the canter?

I have a 17 year old thoroughbred and shes a pretty terrific horse, the only problem i have with her is cantering. here is how i do it, i get her into a slow trot, then i sit a few beats then slide my outside leg back and nudge her, whenever i do this she first goes into a really fast trot and then she goes into a really fast canter. when she does this i tense up immediately and i slow her back down, i know i shouldnt do this but when she goes that fast i just cant help it, i would really like to enjoy her canter but i cant if she is always doing it full speed, any help would be great, thanx in advance :)