John Gill Commentary Psalms 111:8

"They are established for ever and ever; They are done in truth and uprightness." — Psalms 111:8 (ASV)

They stand fast for ever and ever
Not only the covenant and its promises do, but both law and Gospel, the commandments of the one and the doctrines of the other; the law is an eternal law, as to the matter of it, and is not made void by faith, but established.

And the Gospel is an everlasting Gospel, which lives and abides for ever, being established upon the word of God, which cannot be broken; and is continued in the church, the pillar and ground of truth, from which it can never be removed.

And are done in truth and uprightness ;
either made by the Lord according to the truth of things, the moral perfections of his nature and will, and the rectitude of it; or observed by men that truly fear the Lord with great truth and sincerity.