To help your horse make a softer, more balanced canter depart - set him up for it, and then think of allowing him to canter, rather than asking strongly. If you push too strongly, your horse is more likely to push hard with his hind legs into the canter, and begin the transition on the forehand.
In the sport of Eventing, ensuring that our horses are truly fit for our level of competition is one of the best ways to prevent unnecessary injuries.
Horses become incapable of learning when they go past a certain level of fear, anxiety, or stress. This means that if you keep hammering away at the horse, trying to teach them something when they are currently overwhelmed with stress, it will not work.
For the horse, the learning happens on the release of the aid. That means if you don't release the aid, the horse will not learn.
"Don't take the tempo down without revving the engine." ~ Chris Bartle