To refuse to forgive someone else and then to ask God for forgiveness is a kind of spiritual schizophrenia. You are asking God to give you what you are unwilling to give to someone else.
A Christian who refuses to forgive, who chooses to harbor bitterness, who lives in anger and bitterness, that person is bound to be miserable and messed up.
As strange as it may sound, there is such a thing as an “unforgiven” Christian. This is not a statement about ultimate destinies. To be “unforgiven” in this sense means that the channel of God’s grace is blocked from the human side. In particular, it means that you have chosen to hang on to your bitterness and to forfeit your daily walk with the Lord. You would rather be angry than joyful. You have chosen resentment over peace. Your grudges have become more important to you than the daily blessing of God.
If God has already forgiven your sins 100% by the blood of Jesus Christ, how dare you be unforgiving to someone who hurt you? That’s really the issue. How dare you be unforgiving after what Jesus Christ did for you on the cross?
Such a person will be miserable until they forgive from the heart (Matthew 18:21-35).
See Forgiveness and the Lord’s Prayer.