All this have I proved by wisdom: I said, I will be wise; but it was far from me.
All this have I proved - resuming the "all" in Ecclesiastes 7:15; Ecclesiastes 7:15-22 is therefore the fruit of his dearly bought experience in the days of his "vanity."
I will be wise - I tried to "be wise" independently of God. But true wisdom was then "far from him," in spite of his human "wisdom," which he retained by God's gift. So "over wise" (Ecclesiastes 7:16).