From the end. of the earth... — A hyperbolic expression for a great distance. Isaiah (Isaiah 5:26) uses the expression of Assyria, and it would be natural in an exile’s mouth, but must not be pressed to maintain any theory of the psalm’s date.
When my heart is overwhelmed. — Literally, in the covering of my heart, the verb being used (Psalms 65:13) of the valleys covered with corn, and metaphorically, as here, of “the garment of heaviness,” which wraps a sad heart (Psalms 102 title; Isaiah 57:16). (Comp. Tennyson’s “muffled round with woe.”)
Lead me to the rock... — Literally, upon the rock lead me, which is probably a constructio prægnans for lead me to the rock too high for me to climb by myself, and place me there. The elevated rock is a symbol of security, which cannot be obtained without the Divine help. Others take the expression as figurative for a difficulty which it needs God’s help to surmount.