hold your contact, but keep to leg in so they don’t break. you may want to try wearing tiny spurs for reinforcements. if you don’t support that, all i suggest is to keep a strong leg. hope that helped.
I would do half halts every so often when needed. That is a slight pull back of the rein, for a short amount of time. I would do that when she starts to rush off, and until she gets into a collected canter, stop half halting. It is a reward when you stop pulling back, but if she starts to rush again, start the half halting until she gets back until she gets into her collected canter again. Your seat is also very important, when you really sit down on the pockets of your breeches, that is also a signal to her to slow down. Really push down into the saddle. Hopefully those two things combined should help you get her into a slow collected canter. Remember, the half halts are slight, and just enough to get her to slow down. So don’t hold them, do one for about 2 seconds and then when necessary just keep on doing them every stride until you have her the way you want her.