Forgiveness is a doctrine of the church. In the days of Moses, it was a tooth for a tooth; an eye for an eye. But in the dispensation of the church, it is forgiveness. The doctrines teaches that we should forgive the same person offending us in a day up to 70×7 times. That is to say that you must forgive anyone who sins against you, no matter how many times.
We can control our mind. We can should what we want to do and what we do not want to do. We should decide that we will always remember only what is good and try to forget what is bad. We should stop keeping offenses in our hearts. Let our heart meditate on the word of God not on the offenses that people do to us.