Forgiveness

Do you have to work at being able to forgive? Or does it take you by surprise and just come one day? If the other person is contrite, forgiveness becomes a must. However, if the other person is not, does forgiveness become a choice? What if the other person is not aware that he has done something that needs to be forgiven? Is the need to forgive negated? Who decides? The heart or the head? When the head says to forgive, won't the heart just follow? Which one hurts more? not letting go of the pain, or being unable to forgive?