Through forgiveness, which essentially means recognizing the insubstantiality of the past and allowing the present moment to be as it is, the miracle of transformation happens not only within but also without.- Eckhart Tolle
Our ability to forgive or not to forgive is a good gauge of how much control ego has of our psyche. The ego must be the pettiest of all prima-donnas. It typically holds on to a perceived slight with the same tenacity of a dog holding on to a bone.
Forgiveness has been a very challenging spiritual lesson for me. Doing better with it, though. As Tolle suggests above, it's a concept that is really only becomes necessary if we have empowered "the past" to affect our present moment. Tolle's wisdom suggests that "the past" is simply an illusion that exists only in our minds.
I'll repeat that brilliant nugget of wisdom for emphasis: "the past" is an illusion that exists only in our heads. And so is "the future." The present moment is all we have. Try to live your life 100% in it.