I learned a lot about forgiveness and avoiding resentment from someone who had been badly wronged. As the result of someone threatening a witness, and hiding the fact that her husband was sexually molesting their daughter, she lost custody. I was able to get the daughter therapy (which also resolved the custody issue, with the daughter coming home to her mother permanently, and the ex-husband staying far away from the state where she lived).
But, I was wondering what more should be done when I was asked for advice by the woman's attorney on how to seek a pound of flesh from the man who had enabled the ex-husband. In the midst of thinking about that issue, the mother talked to me, asking that I do nothing destructive, and thanking me for my constructive help, including things I had paid for out of my own pocket when money was tight. She was grateful, but she had been healed by the Spirit and did not want to disrupt that process.
With her example, I was able to take seriously the statements that if you are wronged and hold to resentment, you have the greater sin. Over the many years that followed, I spent time rethinking all of my theology on the basis of just what does it imply if I accepted as true that the greater sin is in failing to forgive (much like a friend of mine who treats "be ye therefore perfect" as said in the voice of "you do too know that I mean it.").
First, of course, the rule that not escaping resentment is a greater sin than causing the resentment means that only what we internalize is permanent, and only that which is permanent is significant. When I die, I will not take a cent with me. No material thing has any eternal value. When people rise in the resurrection, they are restored. Nothing done to them is permanent unless they do it to themselves.
Second, closely tied to that, is that in relative terms, most things are only dross. My car, my clothes, my fine twined linens (if I had any, but I couldn't resist the reference), all come from dust and return to dust. We are like children arguing over finger paints and broken toys when we could have so much more. We should ask ourselves, where are our hearts and what do we really treasure?
Third, resentment, the failure to forgive, cankers the soul and cuts one off from God and love. Failure to forgive blocks us from that mercy that would otherwise claim us.
Nothing is worth cutting ourselves off from the Spirit. I found freedom when I realized that what I resented others for was not significant in any real sense, and that the irritation I felt harmed only me. In the end, the only thing I wanted from the memories of those who had hurt me was to be free of resentment so that I would not be harmed further.
I wish I could say I would have found that place without the help of others, but I'm glad for their example and hope that wherever they are today, that God is with them. There may be shades of gray, but some things are absolute. May we escape resentment so that God's grace and mercy always have place.