I find that on very forward going (or unbalanced) horses the most important thing is to merely *suggest* canter transition, not asking for it outright. Just *think* canter as you're trotting calmy along, and merely whisper the aids softly. You are much more likely to get a calm canter.
It's very important that you don't lean forwards when doing this. Many people lean forward exactly as they ask for canter, and this won't exactly help.
Not saying you do that, though, but it's dead easy to do if you anticipate an increase in speed. (Which is exactly what you get if you lean.) By softly suggesting canter you don't actually know when the horse will transition on her own and that probably prevents leaning anyway
She might "need her head" for the transition, and pull the reins slightly - don't get pulled forward as a result.
Once you get the transition keep doing half halts every few strides, and transition to trot or walk as soon as she's losing her balance.
If she's too much on her forehand and stumbling along head over heels you could try walk-canter transitions, although it might only cause a more energetic departure and therefore faster canter, as you have to use more leg if she doesn't know canter from walk.
If she's young it might help to lunge her a bit, so that her own balance improves without a rider thrown into the equation.
Best of luck! Sorry the post got so long