The days of my vanity - This does not imply that those days of vanity were ended (see Ecclesiastes 1:12 note).
The meaning may be best explained by a paraphrase. Solomon states how the wise man should regard the “crooked Ecclesiastes 7:13 work of God” when it bears upon him. He says in effect, “Do not think that thou couldest alter the two instances (described in Ecclesiastes 7:15) of such crooked work so as to make it straight, that thou art more righteous or more wise than He is Who ordained these events. To set up thy judgment in opposition to His would imply an excess of wickedness and folly, deserving the punishment of premature death. But rather it is good for thee to grasp these seeming anomalies; if thou ponder them they will tend to impress on thee that fear of God which is a part of wisdom, and will guide thee safely through all the perplexities of this life” (compare Ecclesiastes 8:12-13). The suggestion that these verses are intended to advocate a middle course between sin and virtue is at variance with the whole tenor of the book.
Destroy thyself - The Septuagint and Vulgate render it: “be amazed.” Compare “marvel not” Ecclesiastes 5:8.