Teach Dazzle the voice command "canter" on the lunge line first. While teaching her "canter," continue riding her (off the lunge) at a walk and a trot so that she doesn't get bored or decide to stop listening. Once she understands the word for canter and will pick it up when I say it on the lunge, try riding her at a walk and a trot while someone is lunging us. When she gets used to that, set her up and cue her to canter, saying "canter." If she picks it up, only have her canter a few strides, then have her walk. Praise her. Try the other direction. If she picks it up, do the same thing. End the session. Do it for longer the next session, and so on. Once she (and I) are comfortable cantering on the lunge, remove the lunge line and practice while riding around the arena. If possible, steal the small end of the arena and close it off so she has a smaller area to work in. Move up to the full arena. Work in the arena alone for a while, then introduce other horses.
Anyways, that's my idea! Thanks in advance for your help!