I couple of things I see in the slow motion (bad shot) video:
1) on the forehand, you are not getting a lot of shoulder rotation. I see the racquet coming back with you bending at the wrist. Try to point at the ball with your left arm so that your shoulders (when the racquet is back) are in line with the path of the ball. In general, I think you are a little too close to the body and too far back on a lot of your forehands. I would try to get more extension on that side (I like to feel as if I am throwing the racquet forward at the ball)
2) you swung too much at the volley - you would have done better to punch straight through the volley (with a short stroke) instead of swinging straight down with a long stroke.
3) In contrast, on many of the slice backhands, you are stopping at contact, instead of finishing long and through the point of contact. Use that follow-through as a guide. The finish helps you aim and determine the amount of underspin. When you finish low, the ball will be shorter and with more underspin, when you finish high, the shot will be hit deeper and with less underspin.
4) On the serve, if you freeze at 16, right at contact, you see that (a) you are not hitting the ball as high as possible, so you are losing power; (b) you are not swinging through the shot (so you are losing power) and are, instead, stopping the racquet at contact.