Somebody on here said something that really stuck with me, and upon trying it out, I found it to be extrememly helpful. It's this:
Think of the down portion of the canter, and really focus on going down with the horse. So, go out and canter around the arena, and feel for the beat in the three beat pattern that is the down beat; when the leading leg is hitting the ground,. It's One, Two, Three, and one , two , three, and . . . . The "three" is the down beat. Start counting the down beat , like this: Down, . . . . Down . . . Down. . . . Down.
When you feel the down beat, really think of your pelvis (pubic bone to belly button), as going down with the horse, into the little "valley" it goes down into with each canter stride. Of course, if you pelvis goes down, you will need to lean back a tiny bit in order for your body to stay vertical (just as a tree stays vertically oriented on a slightly downhill slope.)
Also, think of you feet going downward in the stirrup. NOT pushing the foot down, but letting the whole leg kind of slide further down as if you are trying to go past the stirrup and drag your heels on the arena. So, the whole feeling is of going down with the horse, when he goes down, and then being relaxed enough to let him carry you back up again.
Try this way of counting and thinking the canter and see if it doesn't help.