"Therefore, since we have this ministry, as we have received mercy, we do not lose heart."
For those who have "received mercy," there is never a reason to "lose heart." There will be plenty of times when we are tempted to "lose heart," and to give up--but we should never do so. When we are feeling crushed down, and almost in despair, it seems like there is no hope. But, once God relieves us of our affliction, the bright light of grace shines once again.
It is not easy to hope (continually), in difficulty. But remember: our hope is in the Lord, and not in the things that we desire to see change here (in our lives, the lives of others, or the world generally). If we have mercy, we have solid encouragement.
[Puritan quote of the day: "It argues that our heart is chaste and loyal to Christ, when we can look a temptation in the face, and turn our back upon it." --Thomas Watson, in "All Things for Good"]