Refusing to forgive puts you in a spiritual and emotional prison. Only the key of the gospel will spring you from that prison door. Jesus preached:
For if you forgive others their trespasses, your heavenly Father will also forgive you, but if you do not forgive others their trespasses, neither will your Father forgive your trespasses (Matthew 6:14-15, ESV).
Having an unforgiving heart means that you wish for your brand of justice to take place on them. But having an unforgiving heart also means that our Father will not forgive us–and will dole out His perfect and deserved justice on us.
So, forgive! Forgive because you’ve been forgiven by Christ, dear Christian! Forgive so other may seen the kindness and tenderheartedness of Christ as forgiveness has given you that kindness and tenderheartedness (Ephesians 4:32). Forgive so the world can see how we as Christians love one another, one of the best witnessing tools around (John 13:34-35).