There is too much roll in your lower back. makes one pooch out their backside, and you cannot be on your seatbones in that position.
Sit on horse, drop legs out of stirrups to let them hang. Lift them off he sides of the horse, let them flop back . use your hands to pull the back , thick part of the thigh outward, so that your knee rolls inward. Sit up tall on the seatbones, then put your feet in the stirrups.
Then, like sky said, open the chest and drop the elbows down and back. This will help to bring the thumbs up on top.
your turnout is very nice and horse is super cute..