I agree with the feet. I know you are not that tall but try to make contact with the ball lower by moving forward and hitting closer to the bounce (on the rise) or moving back after it dips (recieving the ball). Ideally practice both.
As for the wrist, check these videos.
Make you sure you have the little 'L' all the time, and try to let the centripetal force of the racquet swinging towards the ball tighten your grip naturally. It is strange to learn, but after some practice, you will have a tight grip at contact without actually gripping tighter by virtue of a correct swing path and racquet head acceleration.