Look, if the problem were you, she wouldn't "turn into a different person." She'd just break up with you. That's what normal people do when a relationship isn't working out for them. There is NOTHING wrong with breaking up--it happens all the time. You start dating someone, for some reason it isn't working, and you break up. It doesn't mean you're "not good enough." It means there is something in HER makeup that you can't fill, any more than alcohol could fill it.
Ever read in the Big Book, or hear at meetings, about the "hole in the soul" that people try to fill with alcohol (before they discover their Higher Power can fill that hole)? Some people try to fill that hole with relationships. And that doesn't work because two people in a relationship should not be filling deficiencies in each other, they should be companions who enjoy and treasure each other.
So she needs to look for something besides YOU (or any other person) to heal that. You can't do it for her, and it isn't because you are inadequate. Whatever she says.