We need to understand that forgiveness doesn’t eradicate the law of cause and effect. Forgiveness doesn’t erase consequences. Take for example, a husband who has an affair and brings home a sexually transmitted disease to his wife. His wife may forgive him, but they still have to deal with the medical repercussions of his adultery and probably with trust issues and other emotional aspects stemming from the affair.
Just because a sin is forgiven, the effects don’t disappear, either for the person doing the forgiving or the one being forgiven. To understand this better, think of the Cross. Jesus forgave those who crucified him, but he still died on the Cross. The mere act of forgiveness didn’t change the physical reality of the action.
This is precisely why Catholics believe in the concept of Purgatory. We may be forgiven our sins, but that doesn’t mean that all the temporal effects are removed. We still have to live with and work through the cost of our trespasses. Neither extending or accepting forgiveness can change that reality. In fact, sometimes we have to live a lifetime with the consequences of our actions, a constant and painful reminder of what we have done.