The psalmist announced a blessing that sounds a lot like Ephesians 1:3, "For the LORD God is a sun and shield; the LORD bestows favor and honor. No good thing does He withhold," -- but then he finishes with, "from those who walk uprightly."
Aah! Isn't that exactly the problem ? GOD has always given good gifts to His people.
He has given many blessings in the Bible, but it's always people who are the weak link!
GOD's promises are only as wonderful as their weakest link — namely: us/me/you.
How can Paul have any confidence that this time the outcome will be any different / better ? Two reasons: These "spiritual blessings" are: "in Christ" & "in the heavenlies." In other words, the "success" of the blessing resides entirely on Christ's performance — not on my performance!