This person doesn't want to be committed to this girl, but the girl has feelings for said person. Person strings her along by telling her what she wants to hear to get what they want from her, then acting like she doesn't mean anything anymore once they get what they're after.
I think the whole "You have a girlfriend, but she's not your girlfriend" line is sort of saying "Yeah, she's not your girlfriend, but you know she should be."
I think the meaning is pretty clear.
This person doesn't want to be committed to this girl, but the girl has feelings for said person. Person strings her along by telling her what she wants to hear to get what they want from her, then acting like she doesn't mean anything anymore once they get what they're after.
I think the whole "You have a girlfriend, but she's not your girlfriend" line is sort of saying "Yeah, she's not your girlfriend, but you know she should be."