Try putting a bat (or something) behind your back foot in bp. This is how my hitting coaches would fix it when I was younger.
With weight on back leg, don't bend it too much, step toward where you want the ball to go ... and only hit good pitches, even in BP.put 70% of your weight on your back foot ... With the extra weight on the back foot it will be nearly impossible to lift it and when you bend the back leg it promotes you driving forward with that back leg.