You must forgive... you must forgive to help yourself move on. You can't go on hating someone. Also, you must forgive to be recieved into the Kingdom of Heaven.
But, as several others said, don't allow this person to hurt you again. You can forgive someone and leave them or let them go. That means that you do so with no malice in your heart, and you are also protecting yourself at the same time.
Our Heavenly Father gives us rules because He knows what is best for us. It is GOOD for us to forgive. However, we cannot allow the person to harm us. He wouldn't want that either. Just forgive this person and then let them leave your life..
Oh, make me Thine forever
And should I fainting be
Lord, let me never ever
Outlive my love for Thee