‘Yet in the daytime YHWH used to command his covenant love,
And in the night his song was with me,
Even a prayer to the God of my life.
I will say to God my rock, Why have you forgotten me?
Why do I go mourning because of the oppression of the enemy?
With crushing in my bones,
My adversaries reproach me,
While they continually say to me,
Where is your God?'
But he thinks back to the days when in the daytime YHWH used to command His covenant love, while in the night time he would remember God's songs, which contained a prayer to the God Who had given him life. They had been happy and secure days when it had seemed that nothing could ever go wrong. Surely then God had not now forgotten him. Thus he determines to buck himself up, and to ask God, Whom he sees as his rock and fortress (no doubt having in mind the craggy fortress in which he is being held) why He has forgotten him, and has allowed him to find himself in this predicament. Why should he be living in mourning at the oppression of his captors, which makes him feel as if he is being crushed. Why should God allow his adversaries to reproach him, as they continually say to him, ‘Where is your God?' (compare Psalms 42:3).
Note the mention of YHWH. His good memories have brought back the thought that God is his covenant God, which is why he speaks of covenant love (chesed). or perhaps it was precisely because he was about to speak of covenant love, that he uniquely speaks of YHWH. The two go together. He saw himself as very much within God's covenant.
The point here is that he will not allow the circumstances to make him forget that God is his Rock, and thus forget about God's goodness, and willingness to act on his behalf.