From the additional notes to Charles Spurgeon's The Treasury of David, on Psalm 119
Verse 77. — Let thy tender mercies come unto me, that I may live. If we mark narrowly we shall find that David here seeks another sort of mercy than he sought before. For first he sought mercy to forgive his sins; then he sought mercy to comfort him in his troubles; now he seeks mercy to live, and sin no more. Alas, many seek the first mercy, of remission; and the second mercy, of consolation in trouble, who are altogether careless of the third mercy, to live well. It is a great mercy of God to amend thy life: where this is not, let no man think he hath received either of the former. It is a great mercy of God, which not only pardons evil that is done, but strengthens us also to further good that we have not done; and this is the mercy which here David seeks. — William Cowper.