Old thread I know but here's how I beat this problem.
1. Hit balls in a net. And don't care less where they are going! Then do the same on the range. The idea is to get a full swing, not where the ball goes.
2. A full finish real swing. Which means I'm hitting through the ball, not at it. Chances are if you are hitting at it, not only is the backswing short, but you have an abbreviated finish.
3. Trick yourself. Set up to the ball, tell yourself you are doing a practise backswing to the top. Throw it up. Wait until you see that clubhead in the corner of your eye. Then in that moment decide to release through the ball to a full finish.
4. Hit a thousand until it's there.
On the course, just trust its there and play to the target, not your swing.