I suspect that a big reason so many Christians find it hard to forgive is that we have twisted Christianity into something more about right/wrong, rather than being in right relationship. Too many Christians are more concerned with making sure they have the right beliefs, rather than how they live our those beliefs. Many Christians have a belief that God is some kind of score keeper making sure people have the right beliefs. It's hard to forgive anyone else, and especially oneself, when you are focused on who is right and who is wrong.