Psalm 39:4 (ESV) … “O Lord, make me know my end and what is the measure of my days; let me know how fleeting I am!”
When we find ourselves burying our true feelings and creating physical and emotional pain for ourselves, then it’s time to talk to the Lord and seek His help. David knew that life was short and that the days would pass swiftly; he also knew that he was frail and that one day would die. He began to measure his days (90:12; 119:84) and saw that they were but a handbreadth (four fingers) and his age nothing in God’s sight. (See 90:1–11.) “Verily, every man at his best state [in his vigor] is altogether vanity” (v. 5) sounds like a statement from Ecclesiastes by David’s son Solomon, and he repeated the thought in verse 11. The Hebrew word translated “vanity” means “a breath, emptiness” (see 62:9; 144:4; Job 14:2; Eccl. 6:12).
One of my Hebrew professors described “vanity” as “what’s left after you break a soap bubble.” In verse 6, he compared life to an “empty show,” with shadow people bustling about, trying to get rich. Busy for what? Wealthy for what? Years later, Solomon raised the same questions (Eccl. 2:18–19), and Jesus emphasized the same truth in Luke 12:16–21. If you measure the length of life, you may become despondent, but if you look around you and measure the depth of life, you are appalled. Life is swift, life is short, and for most people, life is futile. In modern vocabulary, people are living for the image and not the reality.
This is not morbidity. It is a healthy, albeit troubling, recognition that the one certain fact in life is death. In recognizing this, the psalmist parts company from “the wicked,” who in their arrogant self-confidence banish such thoughts (cf. 73:3–12). Longfellow in his “Psalm of Life” echoes what troubles the psalmist: “Time is fleeting. And our hearts, though stout and brave, still like muffled dreams are beating funeral marches to the grave.”
 Wiersbe, W. W. (2004). Be worshipful (1st ed., pp. 150–151). Colorado Springs, CO: Cook Communications Ministries.  Davidson, R. (1998). The vitality of worship: a commentary on the book of Psalms (pp. 131–132). Grand Rapids, MI; Edinburgh: W.B. Eerdmans; Handsel Press.