Sometimes interpersonal situations are out of control and they can't be reigned in or rectified. Knowing when this is the case is difficult, because some of us are intent on holding on to relationships, situations, beliefs, etc., no matter what the cost. We hold on because we believe that is the right thing to do, and to do otherwise is wrong.
I'm here to tell you that not always is that the right thing to do. When people are deliberately hurtful, mean, abusive, detrimental to positive interactions, then holding on can only hurt you, and it only invites more of the abusive behaviors. If you have stated your frustration and honestly tried to maintain a relationship with someone who is far less vested in the relationship's value than are you; like any wise investor, it is time to sell short and cut your losses.
But this is not easy. Making the decision to walk away is very difficult. And once the walking away has occurred, it is a constant source of sadness that one must learn to live with.
I have had to do this for self-preservation. My family is completely dysfunctional, from drug abuse to prison sentences, and everything in the middle. Analytically, I know what has happened to these people, and I can feel enormous sympathy for them. But I just can't participate in their lives any longer because to do so means I get caught up in some strange game of "Let's Pretend We're Not Freaks Despite our Freakish Behaviors". Frankly, no one in my family is normal or mentally stable, and I don't think I am either, having lived through so many horrendous behaviors with these people. But I think walking away has put me on the path toward a much more normal life and I hope I can stay on it.