Immediately after teaching His followers to pray, Jesus gave a warning about allowing unforgiveness to reside in the heart. He said that those who refuse to forgive others won't be forgiven by the Father.
Do not misunderstand Jesus' meaning here. Believers don't lose their salvation when they refuse to forgive. Rather, they break fellowship with God because their unrepentant attitude gets in the way of regular confession and repentance. The Lord cannot ignore sin, and His Spirit will bring wrong behavior to the believer's attention until he or she deals with it.
Forgiveness is an act of the will more than an act of the heart. Often people don't feel like being merciful to someone who has wronged them. But a resentful spirit grows into a terrible burden. The Lord knows that forgiveness is best, even when it is difficult.
You won't deal with a sin until you see it as God does. So assume full responsibility for your unforgiving attitude, and acknowledge that it is a violation of His Word. Claim the divine mercy He offers, and ask Him to enable you to lay aside anger and resentment against the other person(s). As part of the decision to move forward in grace, make a habit of praying for those who hurt you. And if God so leads, seek their forgiveness for your wrong attitude.
A bitter and resentful spirit doesn't fit who we are in Christ. Nor is it healthy to carry an angry attitude through life. That's why Scripture emphasizes the need to forgive. Choose to be liberated from your burden--Jesus promised to make us free when we release our sins to Him (John ).