i have heard that same thing many times but it does not make sense to me. it is true that if i stop harboring resentment or anger it will make my life better regardless of the offenders doings, but when someone causes you tremendous pain and damage, i dont see how that can magically disappear. Even when fully believing that all the suffering is from Hashem and for the good, that won’t stop your stomach from contracting when the person passes you on the street.
and i can understand why Hashem would want me to refrain from wishing him ill or cursing him or wanting revenge as those are all prohibitions, but why forgive someone who is not regretful, remorseful or even willing to admit wrongdoing?