John Calvin Commentary Psalms 38:9

"Lord, all my desire is before thee; And my groaning is not hid from thee." — Psalms 38:9 (ASV)

O Lord! thou knowest all my desire. He adds this, not so much with respect to God, as to strengthen himself in the hope of obtaining some relief from his trouble, and thus to encourage himself to persevere in prayer. It may be explained in two ways: either as indicating his confident assurance that his prayers and groanings were heard by the Lord, or as a simple declaration that he had poured out before God all his cares and troubles. But the meaning is substantially the same: for as long as people entertain any doubt whether their groanings have reached God, they are kept in constant uneasiness and dread, which so shackles and holds their minds captive that they cannot lift their souls to God.

On the contrary, a firm conviction that our groanings do not vanish as they ascend to God, but that He graciously hears them and attentively listens to them, produces readiness and eagerness in engaging in prayer. It might, therefore, be a significant source of encouragement to David that he approached God, not with a doubting and trembling heart, but strengthened and encouraged by the assurance we have spoken of, and of which he himself speaks in another place, that his tears were laid up in God’s bottle (Psalms 56:8). To obtain access to God, we must believe that He is a rewarder of them that diligently seek him, as the apostle states in his Epistle to the Hebrews (Hebrews 11:6). But I prefer the other interpretation: that David here declares that he had unburdened all his sorrows into the bosom of God.

The reason why most people gain no benefit from complaining bitterly in their sorrow is that they do not direct their prayers and sighs to God. David, then, to encourage himself in the firm conviction that God would be his deliverer, says that God had always been a witness to his sorrows and was well acquainted with them. This was because he had neither indulged in a fretful spirit nor poured out his complaints and cries into the air as unbelievers are accustomed to do, but had laid out before God Himself all the desires of his heart.