When we forgive someone for what they did to us, we aren't pretending it never happened, or that it wasn't evil or hurtful. If we did that, we'd be untruthful.
But when we forgive someone, we let go of our desire for revenge, and we also let go of our anger and bitterness. Admittedly, this is hard to do. But often the first step is to realize just how much these things are hurting us -- and the longer we hold on to them, the more they hurt us. Bitterness and anger are like poisons that eat away at our souls, and ultimately we are the ones hurt by them, not the person who hurt us. The Bible commands, "See to it ... that no bitter root grows up to cause trouble and defile many" (Hebrews 12:15).
The key is to realize how much God has forgiven us. No matter how deeply someone may have hurt us, we've hurt God far, far more -- both by our deliberate disobedience and our neglect. And yet God still offers to forgive us -- freely and fully, although we don't deserve it. And we are called to forgive others in the same way -- even if they don't deserve it.
Ask God to forgive you for all your sins, as you open your heart and life to Christ. Then ask Him to give you the power to forgive this person, and to begin looking to the future instead of the past.