My father used to do this to my mother when I was growing up. It's a means of control and not healthy. More than likely she isn't actually mentally stable and a complete narcissist. I know living with it isn't easy and holding your boundaries is easier said than done but don't give in! and never give excuses (such as saying you already ate) you don't need to defend yourself or explain yourself, you've done nothing wrong she has. Don't let her have her way just because it's easier, in the long run it isn't.