The Long and Short of it.

II Corinthians 4: 16 So we do not lose heart. Though our outer self[d] is wasting away, our inner self is being renewed day by day. 17 For this light momentary affliction is preparing for us an eternal weight of glory beyond all comparison, 18 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.
Paul bookend this chapter with this repeated statement: "We do not lose heart." Though he had spent himself for the ministry and suffered repeatedly.  Though life often got frustrating, as he challenged selfish people (like these Corinthians) to allow God to change them from the inside out, and they would not, Paul would not give up. He had experienced the grace of God in Christ, and he had to tell others; he had to let his light shine.  That's because God gave him the big picture.  It was not Paul against the world.  There is a spiritual battle taking place, as the enemy seeks to blind the lost.  This same enemy attacks those on the opposing team with a vengance.  And we are fragile, imperfect vessels.  We should not expect others to immediately respond and change to us - they need to see Christ in us.  They need to be able to see beyond the temporal to the eternal, which we ourselves even fail to see.  When we grasp the big picture that God has revealed to us, we do not lose heart. We trust in His renewing strength from day to day.  We look ahead to where we can only see faintly.