Forgiveness is the removal of a resentment.

A resentment is removed by diminishing the strength of the attachment which was originally opposed, or by reducing the perceived opposition to that attachment.

Sometimes forgiveness occurs a little bit at a time, and sometimes it happens all at once.

One difficulty that is often encountered with forgiving old resentments, particularly those against people who have been in one's life for many years, is remembering the original offenses. People tend to have less patience with family and friends than they do with strangers. I think this is due to old resentments, but remembering the causes of those buried resentments can be a challenge.