Well, first of all that horse is adorable!
I haven't played the videos (I'm at work) but judging by the photos I can tell you this:
-You seem like you have your weight sunk into your heels, very good!
-I like where you're sitting (pelvic angles), you're seat is very good.
-It seems like you have a chair seat in some of the photos, try to pull your leg back. Think about pointing your knee down, that should bring your lower leg back.
-You're hands are in your lap... I have this problem too! Try to extend them and make yourself have a long arm, but be sure to keep your elbows nice and soft.
Maybe others will have stuff to add, but over all you look really good! :)
Every great dream begins with a dreamer. Always remember, you have within you the strength, the patience, and the passion to reach for the stars to change the world. ~Harriet Tubman