"So we do not lose heart. Though our outer selfs are wasting away, our inner self is being renewed day by day. For this light momentary affliction is preparing for us an eternal weight of glory beyond all comparison, as we look not to the things that are seen but to the things that are unseen. For the things that are seen are transient, but the things that are unseen are eternal."
2 Corinthians 4 16-18
So lets break this down: we do not lose heart. Even though on earth we are dying because of all of the sin, in God we are growing. Because all of this death and pain on earth (that will not last long) is getting us ready for a life of joy and faith in heaven that will last forever. So we do not strive and pine about our life today, but look up for the day we will be with God forever.
So we do not lose heart.