I'm no jumper myself, but I find myself hanging around with a lot of them and I take in quite a bit from watching my sister's lessons.
The rein length - compared to what I'm used to seeing, they seemed an inch or two long
looking at the beginning (you shorten them halfway through, however). You want them short enough so that you can hold your arms comfortably high and in front of you with a good contact so that don't have to lean way back or pull your elbows behind you to get said contact. Many riders struggle with this - it's common that they ride into the ring with the horse on the bit and their reins adjusted accordingly. However, once you ask the horse to go, and you can see it in this video, they will raise their heads (jumper-mode, as we like to call it) and the rider suddenly doesn't have the contact anymore! So, remember to shorten your reins before you get going, instead of halfway through the ride like you did at 1:04, and you can avoid situations like at 0:58 (leaning far back in "defence" position) at 1:01 (notice your elbows).
Also, I noticed that you're in a bit of a chair-seat (pause at 1:33). It may be comfy, it is probably this that is keeping him from really moving forward and through. Think of a proper position as being your shoulder, hip, and heel lining up in a straight, vertical line. However, your leg is pushed out in front of you, and you end up leaning back - putting you behind the motion of the horse and more prompt to hold on to his head. If you ride centered over him, you'll be better able to use your seat and legs, stay a little more out of his way, and say "Go forward, I'm with you," instead of "Forward, but wait - I'm not ready! But forward!"
And, if you ride centered and with the horse, you'll be better able to prepare for, prevent, or ride through situations like refusals, fits of bucking or exuberance, et cetera. You'll also find it easier to adjust the horse's speed and stride length, using more of your seat. And don't worry, it's a common problem and may take monthes to overcome, and years to master!
Also, I'm not sure what your instructors preference or the horse's preference is, but I'd almost like to see a light half-seat to stay off his back between jumps. It'll also give you a good leg workout and you'll get an even better seat!
I like the way that you dealt with the refusal - there isn't much you can do in that situation besides staying quiet and just continuing forward. I like that - no loss of temper or a pointless run about, just picking up again and riding as if it never happened. It seemed like you were saying, "Oh, how unfortunate. But whatever, we'll just try again." It almost looked like you were smiling, even!
I can't comment on muh else, but overall - a very effective ride and I see nothing too wrong with it or any mistakes that the average young rider wouldn't make. ~
By the way, Denmark is awesome!