Okay.. you are stiff as a board, you are bracing, your horse is bracing. Thus the serious air and slamming down. If that horse spooks, you will get seriously seriously hurt.
Honestly I would not canter this horse until you loosen up at your trot and get that under control.
This is a disaster waiting to happen!
Work on your seat and your horse at the walk and trot. Once you can do that properly, the canter will be so much easier. You need to looooosen up.
Keep posting videos of your progress so I can keep an eye on you!