the images are kind of small, so I can't see detail. the one thing that stands out is that you have too much roll in the small of your back. your upper body is arched inward, and your elbows look a bit locked.
Just out of curiosity, try sitting for a bit , at the walk, and experiement until you feel your seatbones point straight down. put the reins in one hand , then up to a trot and try to maintain that alignment; seatbones straight down, not pointing backward. just lightly neck rein her for a bit, and keep your elbows bent and upper arm close in to the ribcage.