One of the major causes of human pain is the pain of not forgiving.
"It has been three years, and I have not forgiven my brother. I never will," she burst out in anger. "If I ever get a chance, I will repay him for what he has done. And nobody will ever stop me" remarked another bitter man about his previous employer. "How many time can I taker her/him back?" She says she is sorry; and then comes back and does the same thing to me.
Bitterness is a problem of life, for we humans have an incredible capacity to wound others in the human species. I don't need to prove that we use this capacity to the fullest extent. In the human community, it does not take long to injure another. No human interaction is possible without the accompanying possibilities of pain, suffering, injury and alienation.
Let me ask you, who is at the other end of your resentment?