The children of God have a chequered state, and feel the changes. And, no doubt, with an eye to the wilderness condition of God's people, these things are said. They who live in a moveable tent will not have always the same plain ground, or the same favourable aspect.
Related Commentaries of Psalms 30:6
Psalms 30:6-8
6 And in my prosperity I said, I shall never be moved.
7 LORD, by thy favour thou hast made my mountain to stand strong: thou didst hide thy face, and I was troubled.
8 I cried to thee, O LORD; and unto the LORD I made supplication.