Not to forgive is to be imprisoned by the past, a block to the flow of the divine energy of life. Not to forgive is to yield oneself to another's control. If one does not forgive, then one is controlled by another's impact, locked into a cycle of action, reaction, and revenge. The present is overwhelmed and devoured by the past. Those who do not forgive are least capable of changing the circumstances of their lives.
Forgiveness is ultimately a gift of grace.
We can neither forgive other people or ourselves out of our own volition. If we communicate this desire to forgive in heartfelt prayer, we attract grace.