Hai,

a little addition to what 3ean already mentioned:

4) everytime you punch with your right hand, your left hand lowers. Keep your left hand to your chin/cheek!
5) when moving around the bag, you only move to your right. Try to also move to your left. That will be a bit difficult since your a southpaw, so practice your moving starting slow.
6) when you give more than one punch, you kinda fall forward. Try to keep your weight on your left leg. One for balance, and two: falling into your punch (as we call it here in the Netherlands) you give your opponant an easy game. All he has to do is wait and catch you with an uppercut or hook and your out.
7) when you give low punches, you bent down in your back instead of keeping your back almost straight and bending through your knees. In your case, again you're an easy target voor your opponent.
8.) also when you finisch your punch or punchcombo, take one step back. You stay in one place so your opponent can counter. With a simple step back your (practicly) out of reach for your opponent, without being to far away to attack again yourself.

And again! Always keep your hands up, defense is about the most important thing!
Good luck!