You are sliding forward rather then turning the hips. It's easy to do this fault beacause it is a natural thing to want to move forward to where you want the ball to go. You think the faster I move toward the ball, the more power that will generate.

I have 2 drills that might help. They keeps you in a good position throughout the swing and will promote a turn rather then a slide.

Go here for the walking drill.

And here for the impact position drill.

If these still do not help, come back and we can get more information about what you are doing.