(57) Thou art my portion, O LORD: I have said that I would keep thy words.
In this life, we tend to expect certain "portions" (allotment). But for the believer, we should realize that HE is our portion. Can we, like the Psalmist, say, "The Lord is the portion of mine inheritance and of my cup: thou maintainest my lot."? (Psalm 16:5) If we recognize God as our portion, perhaps we would be more inclined to keep His Words.
(58) I intreated thy favour with my whole heart: be merciful unto me according to thy word.
Similar to verse 41 ("Let thy mercies come also unto me, O LORD, even thy salvation, according to thy word."), there are promises "according to [God's] Word." It is amazing to think that those of us who have obtained God as our portion (through salvation) now have access to God's favor and His mercy - according to His Word.