The last verse is interpreted as a postscript principally because it falls outside the basic alphabetic acrostic pattern which ends in v 21. It is also different in tone, introducing “Israel” into the psalm for the first time. . . . The effect of the postscript is to transform the more individual prayer of the psalm into a prayer suitable for Israel as a nation; the “troubles” (v 17) of the psalmist are analogous to the troubles of Israel.
The metric center of this psalm is verse 10 where the Mosaic Covenant is mentioned which is central to Israel’s worship. Craigie (Psalms, 220) notes regarding verse 10:
Again, it is the covenant character of God which dominates this expression of confidence; as is emphasized in the terminology of v 10; the confidence in God’s “lovingkindness” is linked intimately to the prayer for “lovingkindness” in vv 6-7. But all covenants have two parties, and the lovingkindness of God, the senior partner in the covenant (v 10a), was related to the psalmist’s obedience to the covenant stipulations (v 10b).
Davidson also highlights verse 10 (The Vitality of Worship: A Commentary on the Book of Psalms, 91), “This God is bound to his people and they to him by “his covenant” (vv. 10, 14). This is the first occurrence in the Psalms of one of the key theological words in the Old Testament.”
The Lord’s paths (ways) are loving and faithful for such as keep his covenant. Expressed negatively, his paths or ways are ineffectual only if I refuse to walk in them; however, sin will block my way and separate me from God. A sudden awareness of this reality is sufficient to prompt the psalmist to pray again: Forgive my iniquity, though it is great—so great that he can only hope for pardon from a God rich in mercy. Consequently, for the sake of your name I ask, for you have a name for mercy. I trust in your loving-kindness! Immediately there follows another affirmation that the Lord will instruct the God-fearing man in the way chosen for him, and it will be a way of blessing. Of this the writer is confident. Once again he has come to the bedrock of his faith.