In the beginning of the video, when you are walking with no stirrups, make sure you are bending your elbows. Start by rolling your shoulders back and squaring them, and then bring your arms back and bend your elbows a bit more. You'll find your whole posture to be straighter when you aren't hunching your shoulders. In the trot you've fixed your shoulders somewhat but I think you could still bring them back a bit more. I like that you aren't posting too high. You have a really nice quiet seat and quiet hands, too. :)
When you're cantering, it looks a bit like you are pumping with your seat. I think here you could lighten your seat a bit and tip your body forward more (not hunching forward, but more of a hunt-seat position). I also think it would help you to shorten your reins and push your hands forward out in front of you for more control. You look GREAT cantering without stirrups! It took me a while to realize you didn't have them - your leg position is perfect there.
Pause at 2:30. I love your seat there. That's where your body should be during the canter. Just push those hands forward and shorten the reins and that would be perfect.
Hope this helps! :)
Every ride, good or bad, teaches you something new.