Resentments are the blocks that hold us back from loving ourselves and others. Resentments do not punish the other person, they punish us. They become barriers to feeling good and enjoying life. 
They prevent us from being in harmony with the world. Resentments are hardened chunks of anger. They loosen up and dissolve with forgiveness and letting go.
Letting go of resentments does not mean we allow the other person to do anything to us that he or she wants. It means we accept what happened in the past, and we set boundaries for the future. We can let go of resentments and still have boundaries.
Try to see the good in the person, or the good that ultimately evolved from whatever incident you feel resentful about. Try to see your part. Get clarity on what your boundaries are and be willing to speak up and state your boundaries with others. (teach people how to treat you). Then put the incident to rest.