It happened that while Jesus was praying in a certain place, after He had finished, one of His disciples said to Him, "Lord, teach us to pray just as John also taught his disciples." -- Luke 11:1 (NASB).
Then In Matthew (Matthew 6:9-13) we read the "Lord's Prayer" (or model prayer) as many of us have memorized it:
"Our Father which art in heaven, Hallowed be thy name. Thy kingdom come. Thy will be done in earth, as it is in heaven. Give us this day our daily bread. And forgive us our debts, as we forgive our debtors. And lead us not into temptation, but deliver us from evil: For thine is the kingdom, and the power, and the glory, forever. Amen."
Then in the same breath Jesus added:
"For if ye forgive men their trespasses, your heavenly Father will also forgive you: But if ye forgit men their trespasses, neither will your Father forgive your trespasses." -- Matthew 6:9-15 (KJV)
In the Lord's Prayer, we say "forgive us .... as we forgive", yet we don't realize what Jesus said, in the same breath, that we must forgive others if we are to be forgiven. If we pray the "Lord's Prayer" for forgiveness, we must also follow Jesus' following statement, he made just after teaching the model prayer. We must forgive others before our prayers will be effective. Forgive all, and then pray effectively.