From what I see, your basically arming your forehands too much, I use to do the same thing, I could get good pace on my balls just arming it, but I would run out of juice after 1 set. I've since become a lose arm hitter, and using my legs and body rotation to generate all my pace. I also notice you are stopping yourself from moving forward into your shots. When I hit, it's almost like I'm going to net after my groundies and this has improved my power a lot too.
Forward motion is your friend, try standing a little bit back but move forward into the shot and this will transfer a lot of your body weight into your shot.