Forgiveness is really hard, especially when someone goes out of their way to hurt you. The more un-Godly their act, the more personal their act, the more targeted their act against you - the harder it becomes.
But, as Nukreation said, unforgiveness is a poison. If you do not forgive it will sit there silently poisoning your fellowship not only with other Christians but with God as well. It will help you to become a gossip, perhaps even helping others to become such as well. It will help you "murder" someone in your mind, through the hate you will build up against them. It will help you to not be any kind of witness for Christ at all.
Asking God to help you forgive is a good start.
Remembering how much God forgave you, when you became a Christian, might put it in perspective for you.
Deciding to forgive and then doing it, is what we are supposed to do.
Whether you can forget what they did to you and ever trust them again, is a bit of another story and a little more dependent upon the situation....