Psalm 39 The psalmist, David,  prays, " Lord, make me to know my end, and what is the extent of my days, Let me know how transient I am. Behold, Thou hast made my days as handbreadths, and my lifetime as nothing in Thy sight, Surely every man at his best is a mere breath."
Psalm 144: 4 echoes this thought , " Man is like a mere breath; His days are like a passing shadow." And in Psalm 90:12 we have Moses' prayer, " So teach us to number our days, that we may present to Thee a heart of wisdom."

God alone knows the number of our days.  As we grow older we realize how quickly the days fly by.  How short our time on this earth really is.  We cannot fathom eternity, although it is in our hearts and we long for the day when we see His glory.  We live in the present and regard the past, but our thoughts of the future are limited and uncertain.  We may have a plan for tomorrow or for a month or so from now, but we have no clear view of the days or weeks or years ahead.  For we have learned that everything can change in a heartbeat.    But let us learn to think as David and Moses, in the truth that this life is just temporary and real life is eternal.  
As Paul wrote, "Therefore we do not lose heart, but though our outer man is decaying, yet our inner man is being renewed day by day.  For momentary, light affliction is producing for us an eternal weight of glory far beyond all comparison, while we look not at the things which are seen, but at the things which are not seen; for the things seen are temporal, but the things which are not seen are eternal." 

How do we learn to live this way? These passages give us some clues.
1. We must realize and acknowledge the truth that our lives are transient. KJV uses the word frail here, the Hebrew word is chadel - vacant, ceasing, destitute .  Life is short and unpredictable. We don't like to think about that.  But the Word is telling us that we should.
2. We need to ask God to give us the right perspective. David prayed this.  Moses prayed this.  Paul must have prayed for this too.  Peter makes mention of this truth in I Peter 1:24 and James does also in his letter ( 1:10). Surely this is a prayer according to God's will, His desire for us .
3. We must learn to walk by faith, not by sight.  ( 2 Corinthians 5:7)  It is God Who is at work in us for "this very purpose".  He is preparing us for the day when "what is mortal" will be "swallowed up by life".  We must believe this, trust God for it, and lead our lives according to this truth.

Father in heaven,  I pray for a heart of wisdom, for good courage, and for faith to walk according to Your Word.  I pray that I might not focus on the aches and pains, the troubles and trials, or the fears and failures of this mortal life, but that I might rejoice in the "eternal weight of glory" that You are working in my soul.  O Father, open my eyes to see the unseen and my ears to hear Your truth, to have an eternal perspective and to live accordingly.  Show me how, Lord.  I ask this in Jesus' Name.
Amen