Your lower leg is too far forward in nearly all the photos.. I would work more on two point in the walk and trot and eventually canter. Since you are riding different horses it's hard to sum up what you can do to your position but to answer your question, you have to actively tell yourself to sit up tall.
Someone on the forum was linking this video. I watched it last night and really admired the advice for fixing your position. How To Practice Your Classical Seat In Horseback Riding - YouTube
There it has simple exercises to "fix" your position and gain more contact with the horse.
Watch your hands. I notice on some of them your hands are pretty good, others they go flat and are pulling for contact instead of pushing the horse into it. Open up your chest more and push those shoulders back and down.
Looking pretty good though