Your stirrups need to be about 2 holes shorter, you are standing in your stirrups instead you should be holding yourself up with your legs. You need to keep your heals down, also your back should be parallel with the horse, your back is almost perpendicular. Try sticking your butt out.
You need to release. Don't plant your hands on the horses withers, slide them up to about the center of his neck.
Last, don't sit down so soon, that's why it looks like your horse is hitting the rail with his back foot, in the last picture.
Good luck, and you have a really nice horse.