You need to post "smaller." Your crotch should not come up higher than the pommel of your saddle. Easy on slow, smooth horses, way harder on big, bouncy ones. Posting big for a few strides to encourage your horse to move forward is one thing, but consistently posting that big is another.
You also need to bring your shoulders back. You are "perching" on your hands. Your hands need to come up and you need to lean back. You will probably feel like you are leaning too far back at first, but in reality, you probably won't be sitting up straight yet, lol. Perching is my own bad habit. It's hard to overcome, but always remember to keep your shoulders BACK.
Lastly, or rather before you work on anything else, get that horse moving FORWARD. I want to say this is a lesson horse? Great "confidence booster" horse, I'm sure. Steady, smooth, laid back, am I right? See what you can do about getting him to move forward and off your leg. Use ask, tell, demand and get some energy and impulsion in his gait. You may see if your instructor thinks you are ready for a dressage whip or spurs, or at least other ideas to get the horse moving. It will be hard to work on yourself if your horse has no energy. Posted via Mobile Device