Praying and Waiting Upon The Lord
Psalm 38:15 (ESV) … “But for you, O Lord, do I wait; it is you, O Lord my God, who will answer.”
Usually it is impossible to say why one psalm follows another in the Psalter, but in this case verse 15 may be the reason why Psalm 38 follows Psalm 37. The whole message of Psalm 37 is to trust God and wait for his deliverance, even though the wicked seem to prosper for a time. In Psalm 37 “wait for the Lord” is advice. In Psalm 38 waiting is practiced, and by the very person who gave the advice in Psalm 37.
Waiting is hard to do, especially for us. We live in an impatient age. Someone has said that a hundred years ago, if someone was taking a trip and missed the stagecoach, well, that was all right. He would get it next month. Today we get impatient if we miss one turn of the revolving door.
We can learn what it is to wait upon God from David, for David was a master and model of waiting. When Samuel first approached him, when he was just a youth, he was told that he would be the king of Israel. Yet this did not happen for several decades, and during many of those years David was a fugitive hunted by his enemy King Saul. Even after Saul’s death in battle against the Philistines, David remained a king in Hebron for seven years before being asked to rule over the entire nation. And even later, when his son Absalom revolted against him, David was content to wait for God to rescue him and vindicate his cause.
David was not utterly inactive, of course. He was praying. In fact, he was composing his prayers, which is why we have Psalms 6 and 38 and probably some others. The very fact that he was praying meant that he was leaving the outcome of his sickness and trial with God.
 Boice, J. M. (2005). Psalms 1–41: An Expositional Commentary (p. 335). Grand Rapids, MI: Baker Books.