I couldn’t put it better myself, so I’ll just quote.
Withholding forgiveness “creates a darkness within the soul that begins to turn you into a person that no one wants to be around [pessimistic, bitter, angry, snappish]. In this sense, the offender wins twice – first through his offence, then through your lack of forgiveness.”
So says Baptist pastor Dr Stephen Kim of New York (NZ Catholic: June 12-25, 2016).
Act yourself to make the world a better place and don’t withhold forgiveness until an offender repents. If you're unforgiving, the only one being punished is you.