If you do not forgive, you are only sentencing yourself to prison. So forgive and receive the freedom that forgiveness brings.
Many feel that to forgive means to forget. That is not true. If you forget, all you have done is forgotten. Forgiveness takes place when you know the wrong that has been done and you show mercy and do not hold it against the one who has wronged you. This will not undo the pain and hurt that was caused by the wrong acts of the other. It just means that you are no longer carrying the burden of being their judge. Forgiving someone that has wronged you also does not mean that they will change or even be sorry for what they have done, and that you will continue to let this other person hurt you. You are free to love and be healed. What they do with the forgiveness offered to them is up to them. Some may ever think that they have ever done anything wrong. Your forgiving them will not change this. Still forgive them and pray for their salvation, so that they can experience the One Who came to forgive all who will receive Him, Jesus Christ the Author of Love and Forgiveness!