Choke down slightly lower on the grip. This will reduce the length of the shaft and naturally shorten your backswing. Also, practice your swing in a position where you can fully see your shadow. Watching your shadow, slow down your swing and stop the backswing at the moment your shaft is parallel to the ground. Practice this repeatedly.
It is easy to lengthen your backspin. All you have to do it practice rotation of your midsection. Merely hold your club behind you and gradually swing your shoulders from side to side and soon you will see your swing lengthened.