One awesome way to fix that over the top motion at the start of the downswing is an exercise where you stand about a foot and a half away from a wall with a golf club. You'll swing the club back, and as you start the downswing, let the club drop on the wall and keep it on the wall as you rotate open. This will help you feel the right muscles in your left arm and right shoulder that you need to shallow out the swing. After doing this exercise a few times, you can recreate those same feelings in your swing to improve your form.
