But I say to you, Love your enemies: do good to them that hate you: and pray for them that persecute and calumniate you:
I think that anyone who is honest with themself would agree. Reciting the Pater Noster several times (e.g. during the Rosary) is a continual reminder that we have been forgiven much and are required to reciprocate forgiveness towards others accordingly.
Personally, I see this as something dynamic and not static. That is to say that as we strive to know God and walk with him, our capacity to forgive will inevitably grow.
I have a real struggle with selfishness, and I realize that I fall short of what is required of me. However, I also recognize that as long as I willingly submit to Christ, he will continue to work in me.
This may not sound comforting to those who are really struggling with such an issue, but the work of Christ in our souls must not be measured purely in time, but with a willingness to be docile and to change what He has revealed to us. The key is not closing your heart and will to him!