Letting your shoulders be the driving force in starting your backswing and controlling your arms is one key to getting rid of that reverse pivot. When I first took lessons about four years ago the reverse pivot was a big problem for me. A key to getting rid of it I think is to initiate the back swing with a shoulder turn and think of keeping your hands centered on your torso. If your arms control your backswing and outrace the shoulders back they actually create a forward pressure on your shoulders I think. This is partially what I think creates a reverse pivot.

Stand erect and put your arms straight out in front of you with hands together. Put a club in your hands if you like and do the same. Now rotate back from your waist only keeping your hands straight out in front of you centered on your chest. Don't use your arms at all. Just go back and forth doing this move. That is how you rotate in the backswing but you are bent over in the address position in a regular back swing. Chances are your hand and therefore your arms move first in your backswing and drag your shoulders around. It is your arms putting pressure on your shoulders which help to create that reverse C position.

When you try to incorporate this thought into your swing you are going to think that your backswing is too short, restricted. You won't trust that you have enough backswing to hit the ball like you want. You may try to lift your arms more which will also create a reverse pivot if you lift them too much. But you have plenty of backswing. Especially if you learn to power your swing more with your hip and shoulder turn.