Sit quietly. Close your eyes. Allow your mind to settle and your body to relax.
Imagine yourself beside God after your death. Before you and God place the person you feel resentment toward. It could be someone in the past or the present, living or dead. As you and God are gazing at this person together, see good things happening to him or her. These could include the joy of God at the moment of their birth, God crowning them with crown of gold in heaven, beautiful things happening to them and their family on earth, successes in what they do, deep holiness and surrender to God, immense acts of charity done for others.
By imagining things such as this for that person, you move yourself to a greater love, you melt any resentment you may feel, and you take on the eyes of God for that person, for God only wants the best and holiness for each of us.