"And forgive us our trespasses as we forgive those who trespass against us."
The Fourth Petition began with the word "give." Now the Fifth Petition begins with "for-give." The Fourth Petition has to do with the body, the Fifth with the soul. The physical and the spiritual. Both are important.
The Fourth Petition is like the Old Testament omer jar.* The Fifth Petition is like the holy cross.
The first part, "And forgive us our trespasses," is the vertical part of the cross. And the second part, "as we forgive those who trespass against us," is the horizontal. Pray the Fifth Petition in the shape of the cross.
But let us look not only to forgive those who sin against us, but to be forgiven by those against whom we have sinned.
And remember that the Fifth Petition, like the others, is also a promise. It is a promise that God will forgive us when we ask (otherwise He wouldn't have instructed us to pray this way) and that He will create in our hearts the desire to do the same for others.
The Christian heart is shaped like the cross.
*See "The Fourth Petition" devotion.