The best way to get rid of a resentment is to pray for a person for two weeks. Pray for them and ask that they be given everything you would ask for yourself.
At the end of two weeks, YOU are better, they may have not changed or they may have had a spiritual awakening, but the burden of the resentment should be gone unless you didn't pray with the right motive and intent.
Something will diminish if it isn't fed.