I wonder about your seat, because very often you see a horse jump a bit quick into canter, and the rider's seat tilts forward and lightens - then the horse takes off as a result.
I'd be riding right back on my seat bones, and ask for the canter only when the trot is absolutely balanced and in front of the leg. Make sure you can get a reaction to your half halts, and that your transitions between halt, walk, trot and down again are spot on. Then ask for your canter as you are leg yielding out of a 10m circle. Half halt, half halt, half halt, then think walk and come back to trot. Get your trot balanced again and repeat.
If she takes off and wants to run onto her forehand, WALK. She needs to stay with your seat and not run through it. When you get the walk together, go back to trot, get the trot together, and try your canter again. Most horses don't take to long to get the hang of staying with your seat in canter if you make them walk or halt every time to try to barge thorugh you.