When a horse "falls" into the canter from a racing trot it will be a very unbalanced canter. Sometimes, well, that's just the way it happens, but if you get good at having the horse take a canter from a walk, the horse will be better balanced and will betaking the canter from the outside hind foot as the strike off, instead of falling into the canter on the leading inside leg first.
One of the problems with the horse racing into the canter is that the rider gets thrown off balance too, and ends up leaning forward and this just perpetuates the whole need to fall forward.
You will want to work on being able to stay sitting nicely upright in the canter and in the trot prior to the canter. If your horse is rushing , pull her back, collect her energy and then kind of quickly ask her into the canter. Almost surprise her before she has time to go back to racing. If instead she goes at a faster trot, slow down, gather her and ask again.
It's almost like revving the engine of the car, but holding it back until you just "pop the clutch" (and old fashioned term) and the car surges forward.