Your problem is exactly the reason I don't go from a trot to a lope/canter. When trotting I never let him break to a lope. I can encourage the trot extending it to 12-13 mph without breaking since a canter is not an option ever.
I go from a walk or halt to a lope and he maintains the rythem even if almost stalled until I say walk, again no trot allowed .
For those that go from a walk to a trot and then to a lope are just risking this same problem
You always mention this, and like I've said before, it obviously works for you, so I'm not bashing it, but it's very unlikely that you will ever be able to convince anyone else of this. =] There are tons of people out there, myself included, that can maintain a rhythm at any gait, no matter what the next transition is. Just because you don't want to do that doesn't mean the rest of us are risking it and letting it cause a problem, because really, it all comes down to a lack of training
And to the OP. In my experience, whenever a horse rushes into the canter, it is because he lacks the balance he needs to carry himself through the transition. The fact that he's picking up the wrong lead almost proves my point. You horse needs to work at the trot, circles and serpentines, and TONS of transitions from walk to trot to stop to trot to walk to stop to walk, etc. He needs to learn where his center of gravity is and how to carry himself in such a way that allows him to do whatever it is you ask of him, all the while staying balanced.
Now if he is rushing into the canter as in a "bolting" sense, that's a different training issue altogether. If that's the case, trot to canter to trot to canter to trot to canter, until he realizes that the canter doesn't mean go fast, it just means to canter. Good luck with him. =]