First, what a beautiful horse. So calm. She is clearly enjoying herself.
I agree that you're looking down a lot - you seem to be watching your horse's head where she's going, so maybe the ground isn't level, are there gopher holes or drops and rises? You look worried.
When you speed up you seem to relax more - but overall you need to sit back into the seat - settle your weight deep down into your pelvis and rock your hips with your horse. Also bring your shoulders back and relax and breathe out. THeres a lot of tension in your shoulders and head.
I agree about reading the Sally Swift book to do some visualization about how you should feel in the seat.
I'd also suggest when you walk her around to warm up, you need to warm up your muscles too...drop your stirrups, do some leg lifts and rotate your ankles and do stuff to loosen up. Then do some exercises where you rotate your arms in a big circle one at a time...turn in your seat by trying to swivel your waist/pelvis so you can turn yourself around and look behind you and stretch that. Both sides. You need to loosen and stretch your body.