Without the body loaded correctly during the early stages of the golf backswing, any golfer will tend to over swing as their brains don't have a sense of muscular tension that would signal the beginning of the downswing. This causes them to keep swinging well past parallel in their backswings and the golf swing becomes much more complicated.
This is frustrating because golfers who struggle with this have been told hundreds of times by their golf instructors to just "stop here" or just "shorten it up!" If it were that simple, they would just do it! The key to fixing this problem is to properly load the right leg and core muscles during the backswing during the takeaway for most golfers. This acts as a signal for the brain to reverse directions as the muscles can't support much more load without becoming uncomfortable.
Try this during the takeaway. Try and focus on rotating and "torquing" your right leg and hip by feeling as if they remain stationary as you begin to rotate. If you feel the tension early in the golf swing, by the time you get to the top, you will be unable to swing back any further with your arms. When you do this, you're going to feel fully loaded up by the time your arms get parallel to the ground, so it will make it impossible to overswing.
Practice "over-doing" this for a few days and notice how much shorter your golf backswing becomes!