Your top of swing position looks very solid. You are across the line clearly, and I would guess the cause is the position of your right foot (turned too far to the right) although it is out of the frame of the pic. This lets your hips get out of position, and leads to crossing the line at the top. Square your right foot at address, and focus on keeping the angle of the right leg during the backswing. This will likely shorten up your swing quite a lot. IMO you are better off with a square rear foot position, letting the left heel come up a touch on the backswing if you need to.
You may want to try some practice with a ball under the outside edge of your right foot, that will really give you the feel of the 'right post'. You'll feel restricted, however once you 'get it' you can be 'free' with your over all motion (by letting the left heel come up), but 'solid' with your right post.
