Don't know if this will help but it's worth a try. Get your instructor to look after all other controls other than the rudder. Then try and fly the plane level and tracking to a visible object just using the peddles. Do this down the centre line of the runway etc. I had big problems with keeping the ball in the middle and this method just seemed to hammer the point home of not just using the rudder when there is nothing else to do in the plane.
Regards,
Simon