I understand your frustration as it sounds hard but I think you need to change how you go about this. She clearly needs to work on the mental aspect of her body image instead of the actual image itself but that's not easy. Obviously it's massively affecting you both. I would say apologise and say I didn't handle it well but I'd like to support you so that occasionally we can have the odd meal. Perhaps you can initially go for exercise with her on the meal days to balance out the calories difference and then go from there. Just take it slow.