Forgiveness is important, as is its lack. But before we talk about forgiveness there are useful spiritual disciplines to mention first.
So, let us say that we believe/feel we have been wronged. Perhaps we are angry. Here is a discipline that, under some but not all circumstances, may help as we head to forgiveness (or not), and grow into the full stature of Christ.
1. So, who is the person who has done this? I mean really, who are they? Questions to ask: I wonder how they have come to be this person? What have they been through, how have they grown past the difficulties of their life, whether these difficulties are self-inflicted or inherited? Continue with these and other explorations about the person and the incident(s), and what has led to this alienation between you that you are experiencing. Call this increasing our sympathy or empathy for the one you believe has wronged you.
2. How about me? Where am I in all of this? Think about the lead up to 'the incident(s)'. What can I learn about myself? Am I ready to 'die' to whatever has brought me to be so angry or offended? (E.g. pride, self-importance, or perhaps my anger reveals a part of me that I am actually uncomfortable with, a discomfort that has led to my angry reaction and feelings.) This is not a call to attack oneself, but rather, an invitation to self-understanding, or you could call it sympathy for our own particularity as human beings.
I find that this discipline leads to insight about myself and others, and can let me give up some of what is driving my alienation from the other person. Sometimes, because of this discipline, I can forget and move on.
Not all feelings, not all actions, not every alienation from someone, will yield to the above discipline. Sometimes there is a remainder: I understand something of why they did this, and yet ...
Indeed, plenty of human history will never and should never yield to the above discipline. No matter how much we understand what lays behind or led up to the incident(s), there is no simple forgetting, no easy moving on as though it 'doesn't matter'. (The discipline above can lead us, in some situations, to say exactly that, "It doesn't matter." And this can be real and genuine. To say "It doesn't matter" is to say I've grown, and it - the incident, action, and the alienation I felt - doesn't matter like it did formerly.)
But when there is a remainder, or when understanding, sympathy, empathy, do not cover the sin, we have entered the realm of forgiveness.
The first thing to say about forgiveness must be its asymmetry. Forgiveness is not deserved, it is not earned, it is not a reciprocal coming down off our mutual high-horses and meeting somewhere in the middle. It is full of grace, if you like.
Forgiveness costs us. We venture into alienation, and bringing reconciliation where there is human alienation is neither easy nor cheap. Grace is always costly.
Jesus is, if you like, God's spiritual discipline as outlined above. In Jesus God becomes one of us, one with the human condition. God knows human sin (in need of forgiveness) from the inside, as a victim of it, to the point of betrayal, desertion, torture and execution. God knows us and our sin. God understands us and sin. But the Incarnation (God becoming human in Jesus), death, and resurrection of Jesus are more than (to use my metaphor above) a spiritual discipline. In Jesus God overcomes the alienation of sin. This is God and God's way: God becoming what is not God (in this case sin, godforsakenness, and alienation, see 2Corinthians 5:21 and Mark 15:34) and in this complete embrace of what is not divine bringing reconciliation with what was formerly alienated (us).
And then there is the resurrection. Of course, we should not separate the two, cross and resurrection. The forgiveness that is brought about through God's embrace of godforsakenness (see Mark 15:34) is the resurrected Jesus. The forgiving victim of sin offers forgiveness and sends his disciples out to proclaim repentance and the forgiveness of sins.
There is freedom in this. God's freedom in embracing human sin, and in this freedom carrying our freedom (misused in sin) so that we can grow into our true freedom as children of God.
And there is grace in all this. Undeserved forgiveness (there is no other kind) that is utterly asymmetrical. Forgiveness is always ahead of us; in Christ forgiveness never catches up to us, but we catch up to forgiveness. That is, even though we seek forgiveness through repentance, and hear the word of grace and absolution after, it is forgiveness that arrived first at that place, and we were drawn to it.