Resentment is an extremely bitter diet, and eventually poisonous. I have no desire to make my own toxins. ~Neil Kinnock
“Holding on to anger is like grasping a hot coal with the intent of throwing it at someone else; you are the one who gets burned.” ~Buddha
According to the Oxford American Dictionary, resentment is defined as “bitter indignation at having been treated unfairly. The definition includes the fact that people can harbor resentments going back many years.”
Feelings of resentment are not linked to any particular mental condition but may instead result from the inadequate expression of emotions after a painful experience. They may come from a true, imagined, or misunderstood injustice. A careless comment made by a friend could facilitate indignation and grudging feelings, as could criticism from a boss. Resentment can also be broad and applied to large groups of people, often with drastic consequences; for example, racism and religious persecution often develop from deep-seated resentment. A person experiencing resentment may feel personally victimized but may be too angry or ashamed to discuss the resulting emotions, instead allowing the grudge to fester and be expressed in the form of anger.
A perfect example was given to me by a couple who complained about their younger son, now 38 years old, continuing to harangue them about his childhood. Despite being quite successful at his career, he loudly blames them for all of his troubles. His is not an occasional diatribe but continues every time he talks to them over the telephone. It has gotten so bad that his father, on whom the greatest amount of scorn is heaped, will no longer speak to him. His son is not bothered by this break in communication. Instead, his mother has to hear about all of the past injustices.
Sources:
https://www.goodtherapy.org/blog/psychpedia/
https://www.mentalhelp.net/