Psalms 73:16

When I thought to know this - When I endeavored to comprehend this, or to explain it to myself. The idea is that he “thought” on the subject, or “meditated” on it with a view to be able to understand it. He did not express his opinions and feelings to others, but he dwelt on them in his own mind; not to find additional difficulties, not to confirm himself in opposition to God, and not to find new occasions for distrusting the divine government, but to understand exactly how this was. It was his object to seek and understand “the truth.”

It was too painful for me - Margin, “It was labor in mine eyes.” The Hebrew word rendered “painful,” means properly labor, toil, a burden; and the idea is, that the question was a burden - was too weighty for his weak powers.