Christian forgiveness is an action.
To forgive others means that we will not hold their sin against them and will not bring it up again.
This means in the heart (we not only told the other person that we forgive him, but we really mean it), not just telling a person that we forgive him.
It may be necessary for us to ask for Jesus' help here. For example, Corrie Ten Boom, who spent time in a Nazi concentration camp in World War II, encountered a former guard at her camp. When the former guard asked her to forgive him, she was unable to do so. She said a prayer like: "Jesus, I cannot forgive him. Give me Your forgiveness." Then, she was able to do so.