simple. just tell her: you're a good girl,  you deserve someone better and bid her adieu.
that sentence holds truth for both of you in its own ways. in other words, it is lexically, politically as well as contextually  correct, if you think about it.

on a tad bit more serious note, the keyword, IMO,  is: introspection .
no relationship is bad, it's the people who make it seem so.