Hey, just a few tips.
You are sitting behind the motion. Sift your weight off of your back pockets and forward onto your seat bones. IN this position your weight will be centered over your horses center of gravity and you will be able to stay with her and not fall beack if she were to take some sudden trot steps.
Pick your hands up out of your lap and a bit more forward, Again, this is due to you being behind your hrose. Once you sit up and shift your weight forward, you will be able to place your hands in a better position.
When I say shift your weight forward, think of schooching yourself forward, don't lean forward. In your last pic, you are in an ideal position!
Practice alot of two point. aise up in your stirrups, and sit down. This will place yourself in a balanced position and also strengthen your leg and core muscles.
Engage your abs. Don't hunch and suck your stomach in. One your core is engaged, you will be able to lift your weight off of your butt and onto your seat bones, which will result in a more balanced position.
Again, the last pic of you is very nice!