Many fail to consider the last half of The Lord’s Prayer:
“...and forgive us our trespasses AS we forgive those who trespass against us, and lead us not into temptation, but deliver us from evil.”
Does the author suspect those needing the benefit of the Lord’s Prayer are not genuine Christians?
That being said Christ suffered for Our sin, He was punished for them, even the ones we have not yet committed so that we would not have to pay for them. That payment however was and is conditional, the condition, we must follow Him. His commandment is that we be perfect even as His Father in Heaven is perfect. We cannot be perfect so we have to rely on Grace. Grace is only effective if we want it to be, we must ask for it. We ask for the gift of grace by begging forgiveness for our sin.
We hopefully learn from our sin and don't continue a particular sin, we get over it as we come closer and closer to Christ and His teachings, BUT, the idea is to live His teachings. If we do not live His teachings or at least truly strive to live His teachings then we are not His and His Grace has no affect on us, we are dead to Him.
Wake up, nothing is free. We pay for grace with a contrite heart, with understanding of the horror of our sin. If we gloss over sin saying it isn't a big deal then we gloss over the suffering that Christ gave us in Gesthenemy and on The Cross. That suffering is a result of our sin, we should feel sorrow and horror at what it caused the Savior, if we don't we don't understand and don't truly follow Him. At least in my opinion.