Forgive us our debts (or trespasses) as we forgive those who trespass against us.
Do you remember the guy in Matthew 18 that was forgiven the two million dollar debt by the master but wouldn't forgive the $20 his fellow servant owed him? Sounds fair enough that if he was forgiven so much, he should be more than willing to forgive so little.
However, that is not the way the Lord's prayer reads. It says that we establish the amount. The way and amount we forgive is the way and amount we enable God to forgive us! In other words, if you want to be forgiven completely, then you must forgive others completely.
Lack of forgiveness only destroys you, not the person you are resenting. There is a story about a man who was walking on the beach when a gull flew overhead and landed a dropping of his waste right on the man's forehead. The man shook his fist at the fleeting bird and screamed, "You come back here and clean this up!" I'm not cleaning this up. So he went to work the next day with the bird dropping still on his head. People started wondering about him. He began to smell so they avoided him. The point is that bitterness and unforgiveness is like a bird dropping on your head, it destroys you. Almost always the person responsible for the hurt is not going to come back and make it right. He may not even be anywhere around or may not even realize the pain he caused. Don't let others' actions destroy you!
Forgiveness is a decision, not a feeling. You choose to forgive, to cancel the debt, to "let it go" as the song in Frozen goes. Your feelings may still be agitated, but as you speak to yourself, "No, I forgive and cancel the debt. That person owes me nothing" the feelings lose their force and can begin to settle down. You may be tempted to rehearse the offence and stir your emotions up again. Don't. Take every thought captive and remind your soul (your thoughts, feelings, and will) that you have released that person and canceled the debt. It is finished. Death and life is set before you, choose life. Give and it will be given. Forgive and you will be forgiven. As you make this gracious decision and refuse to allow bitterness to ensnare you, you allow God's Spirit to heal you of the hurt and set you free.
Jesus died for that sin and hurt which someone has afflicted on you. He has paid for it. God is clear in His command. Forgive. Even if it costs you your life. It did cost Jesus His, and He forgave. Romans 5 reminds us that the love and forgiveness of God has been poured into your heart by the Holy Spirit. Perfect love casts out all fear. It is the same perfect love which Jesus displayed on the cross. As you choose to let this love enable you to forgive, God gives more grace to cancel the debt, and His peace fills your mind and heart in Christ Jesus (Philippians 4:6). Be free.