Forgiveness is by no means an easy task to do, especially if someone keeps hurting you or has hurt you severely. As children of God, we have a responsibility to forgive people who have wronged us. Not for them, but for ourselves and God. It doesn't matter what the offense was or the number of times someone hurt you, forgiveness will set you free and not let the person have control over your happiness. We do this to keep from having bitterness and resentment build up inside of us like a cancer. We also do it in order for God to forgive us when we sin too.
Even if you don't have relationship with God, the steps can help you in the process of forgiveness. You must always let in love to start forgiving someone. " Hatred stirs up strife, but love covers all sins." -Proverbs 10:12. Depending on the severity of the offence, time is a key factor into processing through feelings of what someone has done to you. "...forgetting those things which are behind and reaching forward to those things which are ahead." Phillipians 3:13. After moving forward with forgiving someone, including yourself if you've wronged someone, it is good to meditate on scripture. "Forgive us our trespasses as we forgive those who trespass against us." -Matthew 6:12.
Give your hurts over to God so that He can take the burden of resentment and the pain away. As I've been getting closer to the Holy Spirit, I have found it easier to pray for people that upset me or hurt me than it is to retaliate. " But I say to you, love your enemies, bless those who curse you, do good to those who hate you, and pray for those who spitefully use you and persecute you." Matthew 5:44. Revenge isn't worth it and all punishments God gives are perfect according to each offence. God can and does punish those who seek revenge themselves, and this has been true for me even before I had a personal relationship with God. Something bad always seemed to happen to me if I sought revenge and I've seen bad things happen to people who wronged me too, like instant karma.
Depending on how severely someone has wronged you, time and choosing daily forgiveness will eventually take away any hurt and keep it from turning into resentment. As I've seen my own mother die from cancer, I believe all the hate, anger, and resentment she held for people over the years was what caused it, along with being exposed to carcinogens. These hurts and resentments can cause other physical traumas too like high blood pressure, but ultimately, anyone who doesn't forgive others will be worn down in some way. Don't let what someone did to you affect your years down the line. Let go and let God.