I once read a beautiful description of the agony in Gethsemane on a Catholic blog… or maybe I heard it from the Institute of Catholic Culture.
In a sense, the Lord was embodying the voice of all mankind in every part of the human condition. He was truly incarnate, but because He is one of the three eternal and infinite Divine Persons of the Holy Trinity, His sacred incarnation is capable of representing every human person before God.
When the Lord went forth from the Last Supper and passed over Kidron - the sacrifice having begun in earnest - He was already carrying the sins, sorrows, and sickness of the world upon Himself. He was capable of representing everyone and everything, groaning since the Fall, like a terrible boulder crushing Him in the Garden. As a man He saw every sin: past, present, and future, through His human nature’s consubstantial union with His divine nature. Being both God and man, He knew all sin and sickness in a single instant. It was His focus in the Passion. He cried out simply because the dregs of the cup of sin had become so filthy, so wretched, and so sad.
Christ cried out “let this cup pass from me” because that is truly the cry of fallen humanity, which never wants to go with the things God has required of us, that we may live. He did not say this in sin, or as a sinner, but as the High Priest interceding for us. I believe it is the same thing as the cry “My God, My God, why have you forsaken me?” upon the Cross before His glorious death.
The “nevertheless…let* your* will be done, not my own” of Christ is our salvation.
The true drama that was demonstrated by His struggle with that fear of pain-not fear of death, BTW-contrasted with His overcoming that fear by virtue of His obedience to the Father’s will born of His fathomless love for man, serves to show us how deep that love really is.