Psalm 6:2
Have mercy upon me, O LORD; for I am weak: O LORD, heal me; for my bones are vexed.

The truth is, we are weak—very, very weak.  Sometimes it may seem like we hardly have strength to stand in this weary world.  The burdens we carry are too great for us.  The struggles we face threaten to overcome.  And when we look within ourselves, we find nothing that we can use to fight against those things which easily and commonly beset us.  We find nothing in us with which to fight.  We possess nothing on our own that can defeat our enemies.

We see the weakness of human flesh when we return at the end of the day so weary just from living, needing sleep and rest to get from one day to the next.  We have a wrong impulse, and we end up sinning.  At a small slight from someone, we respond in anger.  An annoyance comes, and we give up.  A battle comes along, and we fold.  These are not the characteristics of someone who is strong.

But the Lord will look down upon our smallness and frailty with His tender mercy.  He knows our makeup, that we are nothing but clay and water mixed together, fashioned by His powerful hands.  He knows as well as we do that we can do nothing.  We know how fragile and weak we really are.  But He can heal us and help us, and He is the only one who can.

So in those times when every part of us is weary, at those moments when every ounce of human strength has been sapped from us by this world, all we have to do is cry out to God.  We can plead for mercy from the giver of mercy.  And when we find ourselves at our limits, when we can feel so strongly our own lack, that is one of the best places we can be, because it drives us down upon our knees in humble recognition of our absolute need for God.