The others answered the posting question nicely. Don't be too hard on yourself, it takes time to be good at posting. I agree that in dressage, our weight is more evenly distrubuted down the whole thigh, knee and calf to an anlke that faces mor forward than hunt seat. Don't worry too much if you feel like you are rolled onto the outside of your foot. Try this; move the foot as much toward the inside of the stirrup iron, without changeing it's angle or where on the ball of your foot it rests, just that your foot is a tiny bit closer the horse's side.
Also, thing about almost pushing your weight down into the lower leg which is AIMED toward your horses back feet. In reality, your leg will go downward, but to think of connecting your feet to your horse's back feet is a handy image. I just love those sorts of images.
Also, counting while you post or singing can help you relax and join naturally with the rythm of your horse, the entire purpose of posting.