Albert Barnes Commentary Psalms 119:81

"My soul fainteth for thy salvation; [But] I hope in thy word." — Psalms 119:81 (ASV)

My soul faints for your salvation - The new division of the psalm, which begins here, is indicated by the Hebrew letter Kaph (כ k), equivalent to “k” or “c” (hard). The word rendered “fainteth” here is the same as the one translated “faileth” in Psalm 73:26: My flesh and my heart faileth. The idea is that his strength gave way; he had such an intense desire for salvation that he became weak and powerless. Any strong emotion may thus prostrate us; and the love of God—the desire of his favor—the longing for heaven—may be so intense as to produce this result.

I hope in your word - I trust in your promises, and am sustained. My powers, which would otherwise wholly fail, are upheld by your word, and on that I rely. See (Psalms 119:74).