The first time I taught college after law school, I was a temp for some business law classes. As with all temps in that program, they gave the students standardized tests after my stint to see how much ground the students had lost due to having a substitute.
In what was a first, my students gained ground in knowledge and comprehension over the regular professor, which delighted the reviewer when he explained it all to me. But, the next time I encountered the reviewer, when he was visiting my church congregation, he was virulently hostile towards me. I was in shock at the venom and then discovered that I had been deleted from the list of temps. My response was to just continue to be respectful and polite, which I think surprised him. I've remembered to act that way every time I encounter a similar situation.
Later I learned that he was afraid for his job and for the jobs of his friends, since they did not have J.D.s. Driven by fear he had torpedoed me before the school found out that instructors with J.D.s taught law better than non J.D.s (not at all a certain proposition btw). His wrong drove his anger. Making the choice to put that rather surprisingly savage attack behind me kept me from wasting time. Had I been angry or embittered by the wrong, I would have made a choice to hold to resentment that would only have wasted time I did no have.
In truth, we never have time to waste on resentment.
As for him, his resentment of me hurt him and was his problem, not mine, because I let it go. For me, it was a learning experience. By understanding the reason he had acted as he had, I was taught me to look twice at any time I feel anger or resentment towards someone. I now look inward to see what I may have done wrong to cause myheart to feel hostility towards others and why I am having an emotional reaction to them.
I'm not always in the wrong (though resentment is always wrong), but there are seeds often enough that I'm glad I learned to look.
In response to some questions, I would note that there are only two times when resentment is hard to overcome. First, when we resent those who have wronged us without provocation and Second, when we resent those we have wronged or provoked.
In both cases, resentment blocks us from the Spirit and from the healing love of God, which we need more than any anger, hate or other emotion we may feel that binds us to our resentments.
It can be hard, but only by starting to let go of our resentments can we let the Spirit come in to educate, enlighten, guide and heal us. For mercy, grace and healing, part of healing is letting go of the resentment. While the Spirit brings peace, inspiration and healing, resentment can block the channels the Spirit flows through.
To overcome we need to overcome, so to speak, but it is the only way we have.