The Hebrew verb דָּבְקָה of the first clause of this verse reminds me of a person-to-person defense in basketball. The soul in this case clings in a very active sense, such as basketball defenders do to the person with the ball or anybody who might end up with it. The soul is a veritable shadow of its object of devotion.
I think we might be satisfied to think of God as stationary, meaning the soul is inactive in its clinging. But God is ever on the move, creating new things, transforming old things, working for justice, making peace. For a soul to cling to God means the soul is truly committed. In other places my soul might be told to wait for God, but here, that's not what I picture at all: The waiting is over and my soul is moving and learning and is actively engaged in the world close by God's side.
But lest I think my soul must work to earn God's approval for its actions, the psalmist adds a caveat: Only insomuch as her or his soul is upheld by the Almighty. To be with God I must move with God, but to move with God, I must depend on God.
Uphold me, O God, that my very soul may reflect you in your every movement. Though you are the substance and I but a shadow, I pray that even the shadow you cast through me may be evidence of your presence in the world. Amen.