When I thought... — i.e., when I reflected in order to know this — when I tried to think the matter out, get at the bottom of it. (For the sense of the verb, comp. Psalms 78:5; Proverbs 16:9.)
It was too painful. — See margin.
Related Commentaries of Psalms 73:16
Psalms 73:16
16 When I thought to know this, it was too painful for me;