Even if someone hurts us deeply, we still need to forgive them. We need to do this for several reasons. The first, and the most important reason, is that Jesus readily forgives us and He expects us to do the same for others. The second is that forgiving those who've wronged us frees up our energy. One of the worst things we can do, for our body and soul, is to carry a grudge.
Forgiveness might take a little time, but we need to keep working on it. Put the situation in the hands of God and ask Him to have mercy on that person. We don't know what's in someone else's heart. If they hurt us unintentionally, then we must pardon them. If they did it deliberately, God will demand an accounting. Picture them at their particular judgement. This will help you develop mercy toward them. (It is Catholic teaching that we each will have a particular judgement immediately after our deaths. At the end of the world, there will also be a final judgement, where everyone's sins are revealed.)
If you have difficulty with forgiveness, be patient with yourself. As long as you're trying, you're moving in the right direction. Sometimes putting someone in a mental box is helpful. Imagine they are in a little box, far removed from your present life. Pray that God will change them into the person He wants them to be.
What you ultimately want is for you, and for them, to be able to enjoy being with God in Heaven for all eternity. They won't be the same person when they get there.
Flickr photo by mamamusings