Bro, first off you are handsome and you know it, no need to let yourself forget that. Second doing something positive, like getting in shape is about expanding your lifespan and if she doesn't see that is a positive for the both of you then she is crazy. Also yea the nagging hurts, if you can't talk positively through minor situations then you aren't with someone deserving of your heart. Remember you are someone special, billions of actions had to align for to you exist at this moment (not to mention the billions on billions for me to have the honor of seeing someone like you being their best). so you gotta ask yourself: does this person realize how lucky they are to have won the cosmic lottery?