When you feel responsible for the other person's actions
Is a rape victim responsible for the rape or actions of the rapist?
Is a murder victim responsible for the murder or actions of the murderer?
Is a betrayed, adultery victim responsible for the other person's adultery or his/her actions?
Of course, you know the answer. Ultimately, we are not responsible for the actions of another person.
If you love Christ, possibly you have already sought guidance and counsel. You've been faithful to God's Word, tried your best to reach out, but the other person refuses to change. What can you do?
In Psalm 55, David cried out to God. Somebody close to him, whom he loved and trusted, turned against him. David's relationship with this other person had been severed because of the other person's sinful actions toward him. In response, David sought God's strength, comfort, and sustenance.
"War is in his heart; his words are more soothing than oil, yet they are drawn swords. Cast your cares on the Lord and He will sustain you; He will never let the righteous fall" (Psalm 55: 12-14, 21-22).
Perhaps some aspects of what you're going through with another person are similar with what David experienced with someone he loved and trusted. Understand always that you are not responsible for the actions of the other person.
God is sovereign and sees it all. We simply need to cast our burdens on and trust Him. He'd be the One to make things right.