Verse 9
"Cast me not off in the time of old age; forsake me not when my strength faileth."
The cry of old age: David asks that God not cast him off when the days of old age come, nor forsake him when his strength fails and fades. Precisely in his moments of weakness he seeks closeness.
Verse 10
"For mine enemies speak against me; and they that lay wait for my soul take counsel together."
The cause of his dread: his enemies speak against him, and those who lie in wait for his life take counsel together how to harm him.
Verse 11
"Saying, God hath forsaken him: persecute and take him; for there is none to deliver him."
The enemies' words: "God hath forsaken him" — therefore "persecute and take him, for there is none to deliver." It is a taunt aimed at his very faith: the enemies do not only threaten his body, they try to shake his confidence and convince him that God has abandoned him.
Verse 12
"O God, be not far from me: O my God, make haste for my help."
Against the mockery, David turns straight to God: "be not far from me"; "make haste" to help me. His answer to the taunt is drawing near, not despair.
Verse 13
"Let them be confounded and consumed that are adversaries to my soul; let them be covered with reproach and dishonour that seek my hurt."
David asks that the adversaries of his soul be confounded and consumed, and that those who seek his hurt be covered with reproach and dishonour — measure for measure.
Verse 14
"But I will hope continually, and will yet praise thee more and more."
The turn: "But I" — unlike the enemies, I will hope continually. "And will yet praise thee more and more" — hope does not fade, but becomes an ever-growing praise.
Verse 15
"My mouth shall shew forth thy righteousness and thy salvation all the day; for I know not the numbers thereof."
His mouth will tell of God's righteousness and salvation all the day. "For I know not the numbers thereof" — His kindnesses are beyond counting, and so his praise never ceases.
And so this part rises from the fear of old age and the enemies' taunt to a voice of hope and praise: from "cast me not off" to "I will hope continually." This is the movement of the soul in the Book of Psalms: from anxiety to trust and song.