Robert C. Solomon, a professor of continental philosophy at the University of Texas at Austin, places resentment on the same line-continuum with contempt and anger. According to him, the differences between the three emotions are as follows: resentment is directed towards higher-status individuals, anger is directed towards equal-status individuals and contempt is directed towards lower-status individuals. Resentment will often manifest itself in the following ways: It can be an emotionally disturbing experience which is recurrently felt, or relived in the mind. When the person feeling resentment is directing the emotion at himself, it appears as remorse.
(See Kierkegaard and Nietzsche; also 12-step discussion of Resentment)