Without forgiveness we tend to stack events of the past one on top of another.
It is similar to CO poisoning. Carbon Monoxide begins collecting at the floor level and you never notice it. But, as it continues to fill a room it stacks up until it is at the level that you begin to breathe it in. Then, you experience headaches and stomach problems. The CO continues to stack until the entire room is filled with the poison and you die because there is no fresh air left to breath.
Stacking our sins and negative events eventually fill the room and we die.