Collapsing

Rather than explain what it is, here is the best way to fix it.

Next time you are in the grocery store buy one of those kids kick balls. The ones that are about 8-10 inches in diameter.

Starting with putting or chipping or short pitches; place that kick ball between your upper arms and against your chest. Take the club of choice and with a very small stroke, either a long putt or short chip, practice until you can make clean and repeating shots. The extension you feel in your arms is the feel you want when you remove the ball.

Rather than eliminate the ball and try a full swing, try making longer and longer swings with the ball. Delay any wrist set as you would with longer chips and short pitches. Continue to practice these longer strokes until you make clean and repeating shots.

Finally, try your full swing with the ball in place between your upper arms. You should feel the strong connection of your shoulders swinging the arms and clubs away from the ball together and with improved extension.

Give the downswing time; the longer your arms are the longer it takes for the downswing to accelerate.