Three times in Psalm 42, we read of the psalmist's soul being cast down, and three times this prostration (for indeed, that's what the Hebrew word means) has a response. The first (v. 5) and last (v. 11) time, the response is hope. And in the middle (v. 6) the response is memory.
The person of faith is indeed sometimes brought low by troubles, but we remember and we hope:
For surely I know the plans I have for you, says the Lord, plans for your welfare and not for harm, to give you a future with hope.
Why am I cast down? Good question. And I may not like—or even know—the answer. But I can remember a past when I felt God near, and I can hope for a future when I feel God near once again.
I know there are times when I will be downcast, Lord, whether in body or spirit. I would dare ask to avoid those times. But even more importantly, I ask you that there never be a time in my life that I am bereft of either the knowledge of your grace or hope for a future secure in your love; in Jesus' Name, who taught me to pray: Our Father...