The best way that I would consider the do this is to focus on bag work. Your stance may be the problem, because a stance of a boxer may be a bit off from what I needs to be in order to kickbox. So I would consider looking into your stance. Also it is a good idead to do a crap load of kicks in a row. Like ten 10's as fast as you can, and ten 9's as fast as you can, this will help your balance and stepping. Another good combo to work on this is 1,10 to a 1,2,9. Try mixing up the kicks to leg, head, and body. Throw power into your kicks because you have to get used to handling kicking hard and staying on balance with only one leg on the ground.