Starting with the horse, he has head tilit - coming from crookedness at the poll. This mirrors rider inbalance, or unsteady contact, or a lazy hind leg. He could be a little more over his hind legs but at the moment he's working in a good prelim frame.
For you, the first thing I want to fix if your leg. Drop the stirrups a hole or two, and RELAX your knee. In this photo you are gripping with your knee like your life depends on it, taking the lower leg off which will in fact do the oposite of what you are intending. Your leg shoulder drape around the horse's barel like a wet cloth or slab or meat over a rolling pin. There should be no tension. Your leg is not there to hold you on and give you balance - that's your seat's job.
Your lower back is arched, as you are coming down on your pubic bone rather than seat bones. Sit back, roll your pelvis forward and feel your seat bones come into contact with the saddle, keeping your lower back straight. Right now, you're aiming to give yourself some back problems in the future. A rider's core should take the shock of the horse's motion, not the lower back. Release your lower back, and hold your core.
Your body is twisted to the outside, stemming from your pelvis and going all the way to your head. I'd go back to having a few lunge lessons with no reins or stirrups, eyes closed and really feeling where your body is. When we train a green horse, it is so easy to pick up bad habits ourselves, lose balance and get crooked. Its not doing anyone any favours, so we do need to give ourselves a regular tune up. I find lunge lessons are best for this. Try to picture that your hips are always on the same line as your horse's hips, and shoulders on the same line as your horse's shoulders.
I'd like to see your hands come a little more together, with the elbow allowed to hang. Think of having weights tied to your elbows, or sticking your elbows to your hips. The elbow works as a hinge, the upper arm should not be moving unless for extreme corrections, with only the lower arm, wrist and fingers giving any movement to the bit. Roll your hands up, so that your thumbs are on top and pointing towards the horse's ears.
While you ride, imagine yourself as a Grand Prix Dressage rider. I like to invisage Edward Gal when I ride and feel that I'm losing focus. I close my eyes for a moment, and picture Edward on Gribaldi, he sits so quiet in the saddle, his hands do not move an inch. EVERYTHING is with his seat.
If you can picture someone like Edward while you ride, I think you'll be surprised and find that you improve as you try to mimic that person's seat.