While it is true that unforgiveness involves a lack of thankfulness for what God’s done for us (in paying such a steep price for our own sins), but the true root of unforgiveness has to do with a lack of trust in God. The reason that a person holds so tightly to unforgiveness is because they feel like they are the only one who is concerned about justice coming to that person who wronged them. They do not believe that God is going to ensure that justice happens. God’s Word tells us to give place to wrath so that He can do something about what was done against us, but if we take it into our own hands, we fail to give place to wrath, and can actually hinder God from doing something about it. Do we want the person off our hook, or God’s? Then we need to handle things the way that God has commanded of us in His Word.
One of the keys to living a life of forgiveness, is getting it down into your spirit that God truly cares for you and what is done against you, and will bring justice to those who wrong us as we trust Him with those situations.