"The heart knoweth his own bitterness; and a stranger doth not intermeddle with his joy." - Proverbs 14:10
"For if ye forgive men their trespasses, your heavenly Father will also forgive you:" - Matthew 6:14
One of the most dangerous heart conditions we can have is unforgiveness. If we do not forgive others, we will not be forgiven, and our faith will not work. And everything that comes from Heavenly Father comes by faith. If our faith doesn't work, we are in serious trouble.
"But you don't know what was done to me," people always say to try to excuse their bitterness, resentment, and unforgiveness. Based on what the New Testament says, it really doesn't matter how great their offense was. We serve Heavenly Father who is greater, and if we will handle the offense in the right way, He will bring justice and recompense.
Jesus Christ taught us that we are to forgive those who hurt us, pray for those who despitefully use us, and belss those who curse us. This is hard. But there is something harder -- being full of hatred, bitterness, and resentment. Don't spend your life hating someone who is probably out having a good time while you are upset.
Never try to get people back for what they have done to you. Forgive them and leave them in Heavenly Father's hands.
Power in the Christian life comes from love, not from hatred, bitterness, and unforgiveness.