Get Rid of Over Swing!
As a matter of fact, overswinging encompasses a lot of different pieces and parts, but is generally thought of in terms of taking the club shaft beyond the point where it is parallel to the ground at the top of the backswing. The pieces and parts may include too big a body turn, a collapsing lever arm, cupping or overcocking the wrists, opening the fingers of the lever arm hand, collapsing or totally straightening the rear leg, a reverse pivot and more.
Overswinging is usually a problem because the more moving parts one has and the farther out of position one gets the more difficult it is to bring the club back to the appropriate place at the appropriate time, let alone do it consistently. But it is safe to say that you wouldn't want to teach the golf swing that way, as the odds of consistent success for the huge majority of golfers are drastically reduced by overswinging.
If you check the following areas at the top of the backswing, one-at-a-time, you will discover where the problem occurs and be able to correct it. These things can all be checked by watching yourself swing in a mirror, and certainly a reputable golf professional or attending a golf school could help you with this problem.
Firstly, you can do some things like this: close your eye farthest from the target to test that both eyes can see the ball. The spine angle is still relatively vertical from the face-on view, definitely not leaning toward the target. Your weight is noticeably heavier on your rear leg and that your rear knee maintains at least some degree of flexion - also that the knee has not moved to a position, nor points in a direction, outside the foot.
In addition, the lever arm is relatively extended rather than collapsed or severely bent at the elbow. The hands and wrists are under control, so you need to make sure the fingers of the lever arm hand have not opened and that the butt end of the grip is still in contact with the fleshy "karate chop" pad of the hand.
Also, make sure that the wrist is not dramatically cupped. Most people would focus only on their arms and fingers, but few would pay attention to their wrist. Actually, wrist is also a very important part of your posture on golf swing.