When I said, My foot slippeth; thy mercy, O LORD, held me up. When I said, My foot slippeth - When I found myself so weak and my enemy so strong, that I got first off my guard, and then off my center of gravity, and my fall appeared inevitable: -
Thy mercy, O Lord, held me up - יסעדני yisadeni, propped me. It is a metaphor taken from any thing falling, that is propped, shored up, or buttressed. How often does the mercy of God thus prevent the ruin of weak believers, and of those who have been unfaithful!