Lunging is a great idea, just make sure that you're in control and that it doesn't get dangerous!
Before you ask for the canter, make sure that the trot is quality-- even,
rhythmic, calm, etc. If this problem has been ongoing, she's probably started to anticipate it. If your trot is balanced and even, ask for the canter on the circle -- if she starts to speed-trot, immediately slow her down (halt if you can). While you're making these corrections, take care to be calm and not harsh. When she does pick up the canter, make sure to tell her what a good girl she is! It's going to take a lot of patience, but it'll be worth it!