One of the hardest things to do is to truly forgive someone. It is not easy but it is necessary.
Sometimes although you have said the words "I forgive you" countless times you still end up sometimes feeling resentment when you see the person and think, "they will never change".
It is hard but at the end most people who we hold grudges against does not even know that they have hurt us. So who is the one suffering? Not them.
When they cross your mind and you feel anger and resentment know that you have not truly forgiven them. Take the thought captive and make the active decision to forgive them and say it. It may take you a thousand times of saying it but it is worth it.