There are two sides to forgiving. The first is transforming your attitude toward the person who has offended you, and the second is transforming your relationship with the person who has offended you. Both take courage.
These two aspects to forgiveness take courage because we much prefer to go on clinging to our resentment. That way, we have a scapegoat. When things go wrong, we can say, "Well, that's because of so-and-so and the awful thing he did to me."
It's childish, yet we find the habit incredibly hard to break. Most of the time the best we manage is to "forgive but not forget"—which is not really forgiveness at all because refusing to forget means we are reserving the right to bring the matter up again whenever we please. Holding a grudge—keeping that weapon in reserve—stifles the relationship.
Real forgiveness has no memory. It does not shut other people into the locker of their past mistakes. It makes room for a genuine fresh start. Often, this transformation of attitude is all we need to transform the relationship.