Some excerpts from C.S. Lewis' Essay on Forgiveness:
We say a great many things in church (and out of church too) without thinking of what we are saying. For instance, we say in the Creed " I believe in the forgiveness of sins." I had been saying it for several years before I asked myself why it was in the Creed. At first sight it seems hardly worth putting in. "If one is a Christian," I thought " of course one believes in the forgiveness of sins. It goes without saying." But the people who compiled the Creed apparently thought that this was a part of our belief which we needed to be reminded of every time we went to church. And I have begun to see that, as far as I am concerned, they were right. To believe in the forgiveness of sins is not so easy as I thought. Real belief in it is the sort of thing that easily slips away if we don't keep on polishing it up.
We believe that God forgives us our sins; but also that He will not do so unless we forgive other people their sins against us. [...] If you don't forgive you will not be forgiven. No exceptions to it. He doesn't say that we are to forgive other people's sins, provided they are not too frightful, or provided there are extenuating circumstances, or anything of that sort. We are to forgive them all, however spiteful, however mean, however often they are repeated. If we don't we shall be forgiven none of our own [...]
Real forgiveness means looking steadily at the sin, the sin that is left over without any excuse, after all allowances have been made, and seeing it in all its horror, dirt, meanness, and malice, and nevertheless being wholly reconciled to the man who has done it.
[However] forgiving does not mean excusing. Many people seem to think it does. They think that if you ask them to forgive someone who has cheated or bullied them you are trying to make out that there was really no cheating or bullying. But if that were so, there would be nothing to forgive. (This doesn't mean that you must necessarily believe his next promise. It does mean that you must make every effort to kill every taste of resentment in your own heart - every wish to humiliate or hurt him or to pay him out.) The difference between this situation and the one in which you are asking God's forgiveness is this. In our own case we accept excuses too easily, in other people's we do not accept them easily enough [...] To excuse, what can really produce good excuses is not Christian charity; it is only fairness. To be a Christian means to forgive the inexcusable, because God has forgiven the inexcusable in you.
This is hard [...] How can we do it? Only, I think, by remembering where we stand, by meaning our words when we say in our prayers each night "Forgive our trespasses as we forgive those that trespass against us." We are offered forgiveness on no other terms. To refuse it is to refuse God's mercy for ourselves. There is no hint of exceptions and God means what He says.