See the Matthew 26:30-46 notes; Mark 14:26-42 notes.
Strengthening him - His human nature, to sustain the great burden that was upon his soul. Some have supposed from this that he was not divine as well as human; for if he was “God,” how could an angel give any strength or comfort? and why did not the divine nature “alone” sustain the human? But the fact that he was “divine” does not affect the case at all. It might be asked with the same propriety, If he was, as all admit, the friend of God, and beloved of God, and holy, why, if he was a mere man, did not “God” sustain him alone, without an angel’s intervening? But the objection in neither case would have any force. The “man, Christ Jesus,” was suffering. His human nature was in agony, and it is the “manner” of God to sustain the afflicted by the intervention of others; nor was there any more “unfitness” in sustaining the human nature of his Son in this manner than any other sufferer.
In an agony - See this verse explained in the notes at Matthew 26:42-44.
Sleeping for sorrow - On account of the greatness of their sorrow. See the notes at Matthew 26:40.