Do you ever find the exact answer you were looking for, but didn't know it? Robert Karen's book, The Forgiving Self resonates on many levels. I'll paraphrase but the ideas are all his:
What emotions are at the center of human drama? Forgiveness and grudge. Forgiveness is part of how love works. It can clear a path back from loneliness and alienation. It can also liberate us from the two separate hells: bitterness/victimhood and guilt/shame/self-recrimination. Without forgiveness, human frailty could not be allowed.
Being able to forgive shows our willingness to recognize the humanity in a person who hurt or disappointed us, as well as the gift of seeing our own limitations and complicity.
The grudge begins with hurt, is fed by self-righteousness, and obsessive thoughts about the person who caused the hurt. Facing one's pain is starting to grieve what was too overwhelming to be grieved before. Primitive triggers transport us back to childhood feelings of being unwanted or shunned from the loving circle, and also feelings of gotten what we deserved because we perceived ourselves as unlovable.
A child who loses a parent feels abandoned, bereft of goodness, an unbearable need to have the parent back again, wondering why did he leave, what's wrong with me. Without abundant encouragement and support, this grief becomes too much to bear. Whenever this child experiences a new loss, it hits with the same tsunami force of the first one.
To suffer and be able to move on calls on the capacity to forgive. The experiences that cannot be successfully mourned stay like an "undigested wad of gum" in the soul, a nuclear winter of unbearable, irreparable loss that crushes the soul. This unprocessed pain is the raw ingredient of resentment and grudge.