My partner has sole custody of her daughter 15, who I live with also. 
about a year ago, she left me to get back with her ex.  Who isn't the child's BF but treats her like one. 
she said this was for the benefit of her daughter. 
 

silly me ended up taking her back when she said it was all a mistake. But I can't help feel stupid, when she still tries and talks to this ex. 
 

But whenever I say anything, she goes down my throat that she's only communicating for her daughter.