I would estimate of the thousands of lessons I have given to amateurs at least 50 percent of them swing the club too fast. I have found there are basically two reasons for this.
First amateurs have not had proper instruction on the mechanics of the golf swing. They haven't learned to properly coil and uncoil the bigger muscles, but rather make an instinctive quick flailing motion with their hands.
The second reason for an overly fast swing would be anxiety. This anxiety causes amateurs to rush their swing in order to see where the shot goes or just to get the swing over with. In either case the results are almost always bad.
Here are few mental and physical exercises to help you slow your swing down and give it time to function properly.
1. Make a slow practice swing. On the practice tee before hitting a full shot make two very slow practice swings at no more than 75 percent of the speed of your normal swing. Then step up the ball and try to hit the shot at the same pace - say 75 percent of your normal pace. You may be surprised by hitting the most solid shot of the day.
2. Keep your backswing and downswing speeds the same. Many players who swing too fast may not be quick taking the club back, but get extremely fast coming down. Therefore their backswings and downswings don't match. When this happens you can't fully utilize your lower body in the downswing. Make an effort to keep your backswing and downswing speeds the same by swinging your arms down and through the ball at the same pace which you brought them back.
3. Hit shots with your feet together using your 6 iron with the ball teed up slightly. You'll quickly learn that you must swing in balance or you'll fall over, and that in order to stay balanced you must make a slow and smooth swinging motion. Don't worry how far the shots go although you may be surprised at their distance because you hit the shots squarely.
Amateurs can eliminate a lot of faulty shots by simply learning to slow their swings down. With practice these exercises will help you realize that by swinging in a more leisurely fashion you are allowing the centrifugal force built up in the shaft and clubhead to release through the impact zone.
A good thought is to swing the clubhead through the ball rather that hitting at the ball. Try these drills and you'll soon be surprised how solidly you strike the ball and how much farther you hit it.