If you suffered pain from somebody, the person's name and face are put into your mind with the feeling of hatred and then you have ill feelings against him. So, you are distressed by the hatred that have harbored in your mind even though you don't want to hate the person. To escape from such distress, you must put yourself in the person's place first and put only good images in your mind. And when you think about the good points of the person and show him the deeds of kindness, hatred will gradually be changed into love.
"[Love] … bears all things, believes all things, hopes all things, endures all things." (1 Corinthians 13)