How can a person be assured that upon forgiving, he or she will no longer experience any more negative emotions such as anger?
I think you are expecting too much from the forgiveness process. As imperfect people, we do experience some left-over anger or sadness and this can rise and fall depending on circumstances (such as a new incident that reminds you of the previous injustice). Therefore, I would encourage you to lower your standards for having some negative affect. As long as the negative emotions are not controlling you, but instead you are in control of those emotions, I think you are doing well.