You're bending your left arm alot in the backswing. Try to widen your takeaway, and try to take the club really high after that. That should straighten (don't make it stiff, though) your left arm up.

Also, you're casting. I don't know if you do this conciously or what, but there are many ways to fix this. You can try dragging the butt end of the club towards the ball, and there's the leadbetter pump drill. I suggest you do a search and you will find many fixes and swing thoughts to help you correct this mistake.