Again, I am going to pass on a lesson I learned from The Peavegiver.
We all know that Christ atoned for our sins. That He paid for MY sins and YOUR sins. But do we know that He paid for the sins of others? The sins committed against us? Do we really know that?
The reason it is a greater sin not to forgive is that it means we don't adequately understand Christ's Atonement. It as if we are saying, "I know that you paid for my sins. Thanks, by the way. But what this person did was just TOO MUCH. The price you paid doesn't cover what he/she did to me. Your Atonement is insufficient for this crime."
Christ has taken the sins of all onto Himself. He has said, "Let me pay for this." In so doing, the balance of justice is with Him. In the classic creditor/debtor/mediator scenario, Christ is the mediator, while we see God as the creditor and us as the debtor. But let's look at this a bit differently. Let's put ourselves, the offended, in place of the creditor. Somebody "owes" us something. The person who has wronged us is the debtor, and Christ is still the mediator. He steps in to pay for what the creditor has taken from us.
If someone owed me $100 and wasn't able to pay me when they said they would, I'd be mad. But if a friend of that person came to me and said, "Here's your $100. From now on, they'll owe me the money. But you take this and don't worry about it anymore," I'd be okay with that. It's no longer my problem. I wouldn't refuse the money just because it didn't come from the debtor, because I trust that the mediator who gave me the money would expect repayment from the debtor.
So when Christ says, "Listen. I've paid for this. I know they hurt you, but let me take care of it. Give your burden to me. I'll make sure justice is served, but you don't need to worry about it anymore," we must accept His offering. He is a completely innocent party, but is taking the guilt on Himself. We know that he didn't actually commit the sin, but He's acting as if He did, so we can forgive. It's easy to forgive someone who didn't do anything wrong. What is there to forgive? Nothing. Nothing.
If we don't forgive those who have sinned against us, it's like we're not forgiving Christ.