Any suggestions for slowing down a canter?

I have my Norwegian Fjord in a Maginues bit, but that is illegal in dressage, and I've tried a pacifier. She is so much better in a Maginues, so what would you suggest I do to slow her canter down and get her under control. I do walk-canter and halt-canter transitions, but she just gets hotter and hotter as we go. I'm trying to compete in Training level, but I don't do as well at the trot because I can't warm up, because she gets to the point where I can't control her. Please help me??