dysfunctional relationship

My fiancé knew exactly what to say to wound me and knock me off balance. A scumbag may fool your heart, but he can’t fool your ability to trust your gut.

I'd like your opinion on why a smart, formerly-independent woman can't break ties with someone who I know is emotionally manipulating me, making me question myself, uses me as an option at his convenience, is a womanizer, and admits to being a narcissist?