Resentment

Resentment is like
taking poison and waiting
for the other person to die.

~ Malachy McCourt

It seems to be part of human nature to be resentful of people or situations that have hurt or angered us, of having been "done wrong." We may ruminate about what happened and talk about it in a repetitive way, with no movement toward resolution, like a dog who grabs onto a bone and shakes its head back and forth, over and over again.
There IS value in looking at how you have been hurt, at who or what has been hurtful to you, and allowing and exploring the associated feelings for as long as they are present. This is an experience of feelings moving and flowing through you; after a time, there probably will be less of a charge about whatever happened. What may have initially felt like an open, infected wound, now has a protective scab or has healed into a more or less visible scar.
Resentment, on the other hand, is in reality a refusal to feel the emotions associated with the incident that created the hurt or anger. It is a stagnant experience, a repetitive, often compulsive need or desire to keep the experience alive, like a video looping around in your head.This keeps your stress and flight or fight hormones engaged and continually firing, taking away precious energy from other parts of your life.
The desire to hold onto the anger or hurt about the person or incident is often an attempt to not let that person off the hook; as though letting go of it will undeservedly free them. Yet as the quote above so aptly states, resentment is a poisoning of yourself in an attempt to punish the other person ( who may or may not be aware of how you feel or how they impacted you). 
Again, it IS important to feel and work through your feelings about whoever or whatever was hurtful, but carrying around  the equivalent of a backpack of heavy stones  so that the perpetrator will suffer, really doesn't work, and only serves to perpetuate your pain and suffering, rather than alleviate it.