@mousy,
When people sin it hardens their hearts so that it makes them feel as if they arent worthy of being forgiven or are "too dirty" to be forgiven. We continue to feel guilty and it makes us feel like God will never forgive us.
Remember that The Bibles greatest prophets were murderers. Moses, Paul, King David were all used by God after commiting great sins.
God forgives if we only ask, turn from our ways and follow him.