This is one of the most common questions of someone who starts reading the Bible. I found a lot of explanations, however, none of them is really creating a reasonable answer for why Jesus said that. Also if Jesus knows that He has been sent to Earth to die on The Cross to save all mankind, why is He sad in that garden when he begs His Father to remove this duty from Him: "Father, if you are willing, take this cup from me; yet not my will, but yours be done." - Luke 22:42.
I know He said that the Father's will should be done, not His will, however, he said to His Apostoles countless times that "The Son of Man will be given to the sinful men to be crucified". (Not an actual quote, just my translation to English from my language). Now, don't think I accuse Jesus of unfaithfulness or I deny His Divinity. I'm orthodox so I believe that God is the Father; God is the Son; God is the Holy Spirit.
All the explanations I found on this question revolve around the idea that His Human Nature was fighting with His Divine Nature. The part of Him that was God was saying: "Father, forgive them for they don't know what they are doing", the human part of Him screamed in pain to God that abandoned Him. The God Part from Him wanted to complete His ministry of salvation and to be crucified, the part that is human of Him was scared and asked to give up. However, I don't find this explanation of the 2 Natures: "The Divine Nature and The Human Nature" in Christ that were fighting each other.
I expect answers from the Holy Trinity perspective.