Anger, hurt, resentment...
A grudge hurts you more than the person you're holding it against. You are the one killing your happiness by holding onto those negative emotions.
Even when the other person is wrong, even when they should be the one coming to you to apologize... you cannot let this resentment pull you down any longer.
Choose forgiveness for this other person (even if they won't accept it) to give yourself the peace and happiness you deserve.