We perfect the cause of Christ when we forgive. That's why He came to Earth. Our ultimate goal as Christians is to be more Christ-like. We carry on His work as we strive to become better, more loving people. That includes forgiving others, even when it's hard.
The road to redemption is paved with small stones of progress. This progress is made up of consistent, deliberate decisions for the faith.
Jesus paid off the mortgage on my soul and I'm forever grateful.
Ephesians 4:32 (KJV) And be ye kind one to another, tenderhearted, forgiving one another, even as God for Christ's sake hath forgiven you.