Christians are commanded to be forgiving people.
i think we often pay lip service to the idea of being forgiving, but spend plenty of time justifying our own personal bitterness and grudge-bearing. "Well, yeah, that's true, but in this case God will understand my bitterness because this person just plain doesn't deserve my forgiveness." The trouble is God never said He would just understand it if i don't do what He asks me, and He never said that anyone had to deserve my forgiveness to get it.
God flatly commands me to be forgiving (Eph 4:31-32; Col 3:13; Heb 12:14). And He plainly says that bitterness and resentment and grudge-bearing is wrong (2Tim 2:24; 1Cor 13:5; Col 3:19). i'm either going to obey God or i'm not. Trying to find various ways to justify or explain away my bitterness and unforgiveness is really just choosing to disobey God.
That means i need to just 'fess up to my unforgiveness and get rid of it. i need to just get real about the fact that i am bitter or resentful or holding a grudge, and i have to admit that this says more about my heart and character than it does about the person i'm still angry with. My unforgiveness displays my un-Christ-like heart, not the person i'm holding a grudge against.