Actually, I believe strongly in what is done, is done. What is the point of holding onto grudges?
If another individual will not apologize to you, can you just let it go, or, do you forgive? However, if you feel truly wronged. You may not wish to put yourself in the awkward position of seeing that person, dealing with hurt feelings or, waiting for an apology that may never happen. What I do know to be true is that percolating with that grudge is unhealthy and can make you sick. And, what good is that?