I loved this answer from a reader's question on Liguorian, a religious publication.
"When I pray the Our Father, I have no problem praying “and forgive us our trespasses,” but I really struggle with “as we forgive those who trespass against us" Can you help me?
You have identified a teaching of the Lord that is truly astonishing both in the simplicity of its words and in the difficulty of the response it demands. Our human experience of this almost- daily struggle is something we should be familiar with.
The essential teaching about mercy and forgiveness contained within this prayer defines what it is to receive mercy. Jesus taught us that the full experience of “mercy cannot penetrate out hearts as long as we have not forgiven those who have trespassed (sinned) against us”. In other words, we cannot experience the fullness of God’s mercy unless we, too, are merciful, as it says in 1 John 4:20: “Those who say, ‘I love God,’ and hate their brothers or sisters are liars; for those who do not love a brother or sister whom they have seen, cannot love God whom they have not seen.”
All Christians should have the same struggle when they pray the Our Father, but some never think about what the words actually mean. It’s through the power of God’s grace that you have arrived at a point in your spiritual journey where it’s impossible to just rattle off a bunch of words. “Grace is favor, the free and undeserved help that God gives us to respond to his call to become children of God... partakers of the divine nature and of eternal life”. The difficulty you’re having in the prayer is in and of itself the profound grace you need from God.
Be confident that the Lord who has given you the grace to come to this awareness is the same God who will give you the grace necessary to practice what you pray. That gift of grace will come to you in the moments it is needed. ---Fr. Tom Santa