Dont think about it too much. If you can only manage three strides only do three strides and pull back to trot before going back to canter. (a good cheat is to get someone to lunge you, tie your reins up and hold on to your saddle to get the rhythm.) I found when I teach that sometimes lengthening your stirrups by one hole allows you to sit deeper. Don't give up and good luck. Let us know when you've had your first canter across the fields.