When I thought to know this; literally, and I meditated, that I might understand this. A process of careful thought and consideration is implied, during which the psalmist tried hard to understand the method of God's government, and to explain to himself its seeming anomalies.
But he says, It was too painful for me. He did not succeed; he was baffled and perplexed, and the whole effort was a pain and a grief to him.