HOw long have you been doing canter work? It does take some time to get used to it. Things like cantering without stirrups will help, but if you are not ready for that, it's understandable.
I sometimes tell myself to think of my heel being mentally connected to my horse's hind feet. That helps to keep the weight down and back.
Also, be sure that you arent collapsing your upper body around your belly button, in a C shape. Ride proud, with your "heart" being the first thing that breaks the air as you go foreward.
AND, it just takes time, so don't sweat it too much!
Where in Washington are yoU? Wa. State, right?