Keeping the weight on balls of feet
You have no balance whatsoever in the heels of your feet so therefore you must learn to keep the weight in the balls of the feet. Whenever you step or change stance try and keep the weight in the front part of the foot. Rocking back onto the heels means you are vulnerable and open to attacks.
A tip when free sparring is to watch when your partner is flat footed or off balance and use that instant to attack.