I agree with Saddlebag. The horse is bracey in the neck, so it's a trick to get him to accept the bit and contact , without him wanting to giraffe it.
First, though, your seat is not really solid. I think this is what you are seeing in the video, and kudos to you for noticing that. some people would not see that..
I see you knee move around a lot and when the horse transisitioned downward, you fell forward a bit. you are doing more posting than he is doing. I mean, he should be lifting you, and you seem to be doing more work than his movement would indicate is right.
I would lower your stirrup one notch. The saddle may be too small for you, if you look where your knee is, it's too high, and too far over the flap. Like Saddle said, you need to come down more onto the middle of the saddle, not against the back, and having a slightly longer leg, which is more solid (less knee movement) will help a lot with this.
Your hand is reasonably steady and you look up really well and look focussed and to be enjoying yourself. gettting that more solid leg will also reduce your tendency to roll forward onto you pee-pee (forgive the kiddie word), and you won't fall forward when he drops down a gait.
He needs a bit more "umph" in his trot. I bet when he gets moving he is a very handsome mover.
Good luck on your show and thank you for sharing.