Get a pair of spur straps and attach them from the stirrup to the girth. You are physically unable to wiggle your lower leg. Old fashioned, but worked for me.
i had the exact same problem and i found being lunged really really helped, it takes your mind off of the horse and trying to keep it steady/stear and it helps you concentrate on yourself. also try riding on/off the lunge without stirrups, strengthens your core muscles and helps you flex and lengthen your leg naturaly.
leah