The person we resent probably isn't thinking about that resentment. We are not hurting him. But we are hurting ourselves by carrying that negative baggage. The Big Book says resentment is the "number one offender." It causes stress, spiritual bankruptcy, and emotional paralysis. We need to change from resentful people to forgiving people.
Pray that the person we resent heals. Ask your Sponsor if you are ready to do a 4th step.