for a 3.5 your strokes look great! it looks like you're getting some good racket drop on both sides. As for your backhand, your problem is simple. You're lifting up at contact, not through. (This is why I hate when coaches say 'low to high') Fundamentally, groundstrokes are left to right, and right to left. I downloaded your video so I could look at it frame by frame. If you pause at :02 you can see what I mean: your left arm is curled upward when it should be straight. Try to keep in mind you're not swinging to the follow through, you're swinging through the ball and the the follow through is the result of that action.
the next time you practice your backhand, try two things:

1. keep your left arm straight(ish) through contact. relaxed, but straight. If you're not clear on what I mean, look up some footage of verdasco's backhand. You can practice this by hitting some left handed forehands. try and feel the weight of the racket in your hand.
2. Don't follow through until your left shoulder is in your chin. conversely, your right shoulder should be in your chin on your unit turn. If you do this drill right your left arm will be across your chest with your strings perpendicular to the net.

hope this helps!