I first watched the video right when you first posted it and was too lazy to type the novel that I'm going to write now then. So just prepare yourself haha.
First, I'm just going to point out some of my observations and then suggest how I personally would fix them. If you're in a "read-y mood" I would suggest ::: Sustainable Dressage - - Welcome to my Site about Sustainable Dressage! :::
. Just read it with a grain of salt, the lady is a little bit bitter and traditionalist, and pay attention to the pictures of "classical" versus "contemporary" dressage.
The first thing that I see when I glance at your riding is a quiet picture of a lower level horse and rider. But then upon actually watching your seat and riding and how your horse responds to it, I see a stiff and forced rider on a willing horse. Every single step you are driving that horse with your seat, which is why he is so far out behind your leg. You get very "fanatic" as my coach would say in your core and then stiffen your limbs to reduce movement when in reality we want things the other way, and much looser and harmonious. Your horse is swishing his tail and nodding his head in protest, but your cocked wrists and locked backwards elbows keep his head and neck pulled in.
The biggest thing that you need to fix, and it is a gradual process, is trust of your horse. Once you trust your horse enough to do his job freely, then he will be happy and perform his best, same with any horse. Trust him to know how to walk, trot and canter. He knows how to do those gaits a whole lot better than you do.
I would like to see your lower leg come back far enough that when you stretch your knees down towards the ground, your heels become weighted underneath your seat and your calves slightly close enough to keep your knees from gripping and your legs from swinging. Then you need to focus on keeping your seat at the very front of the saddle, and allow the horse to move under you in his own way. Try to keep your seat falling in the exact same place on the center of the horse, every time. Now look up and balance your head on your shoulders, it should require no muscle tone, only balance to keep your head on your shoulders. Now relax your elbows and bring your hands up and together. It is useful to bridge your reins or ride with a crop under both thumbs to keep your wrists from cocking and your elbows from pulling.
Now as you are riding in this position, every step think about keeping you and him balanced. First focus on keeping all four feet exactly under him, and you landing perfectly in the center of him every stride. Once you have this balance then try to make his neck and spine longer, not through stretching forward and downward, but by letting him stretch straight into your contact and his spine longer and longer straight (like the horse on the front page of sustainable dressage). Don't worry about keeping his head down, only about getting his neck as long as possible.
Good luck in your dressage journey and hopefully your lovely horse takes you far :)