It sounds like you are both very hurtful towards each other. If this is how your relationship has always been, what else would you expect? Have you tried to sit down calmly with her and talk about it instead of lashing out back at her? Is there anything in particular that seems to bring on her comments about your weight? If you haven’t tried to sit down calmly and explain how you feel when she calls you fat, you need to do that. She may believe that by telling you that you are fat, you will do something to change that. Some people don’t understand that acting like that only makes matters worse. If you talk to her about it and she doesn’t change, politely tell her that you need some space from her and don’t talk to her about it anymore. If you are with her and she brings it up, leave. Don’t feed into it by continuing to argue with her about it though because that won’t solve anything and it will only make you feel worse. Good luck!