@core- I will work on that thigh issue. This boy is huge and he felt like I was sitting on a couch. Like a little kid riding a big horse! My instructor was joking before I mounted "you ready to do the splits?". I was having a hard time figuring out my lower body position in general- don't know if it was the saddle or the size of the horse? Yes, we did talk about seat bone pressure. I felt like if I did bring my knee down, my ankle didn't have the room downward to compensate because of the stirrup, you know? Am I wrong? Like in the second pic, my knees are more downward than the third, but I feel like my ankles look forced down? I kept wanting to roll my thighs so the inside was along the horse, not the bottom, but I felt cramped in the stirrups.
I agree that your stirrups were a bit too short for your leg to sit in the classic dressage position. BUT... that's a big, wide bodied horse, and Friesians are known for having a trot that will throw you around a bit, so I can see starting with the shorter stirrup until you're comfortable with the saddle and horse.
I'd give it another lesson or two, and if you feel secure on the horse (not using the reins for balance), then ask to have the stirrups let down about two holes.
The only thing I can tell ya is to practice at home. Sit up tall, sit directly over your seat bones with weight evenly distributed, point your knee caps down toward the ground, and let your weight sink through your heels. Tighten your abdomen muscles until your lower back flattens, and lift your entire rib cage up. This allows your hips to follow the movement of the horse and absorb the movement without having it affect the rest of your body.
Study some of the video's on youtube for dressage rider positions. Then practice off horse to get the muscle memory down.
I would love to see some video of that horse. He looks like he'd be a blast to ride. :)
And again, you look great in the pictures. A few minor tweaks to your position, but far, far better than most beginner dressage riders I've seen.