I agree with the above posters..
I weigh 85 pounds and I can sit my horses canter.. granted, he has an awesome smooth canter. haha So you can do it too!! ;)
It takes a lot of time, and I think that once your understand the tempo and motion of your horses canter.. it will make it easier. Pretend your seat is glued to the saddle, and sit really deep and tall. Get used the tempo and rhythm of your horse's 1,2,3 1,2,3 canter beat.. and relax your seat while lengthing your legs.
Maybe practice trotting without stirrups and such..? It helped me learn to lengthen my body and relax, plus you have no stirrups for support so you have to use your seat to stay in balance and on the horse.
Don't worry! It will get easier!