Actually the answer is in the Catechism in a round about way. Christ’s human nature was real, he had a human body ensouled by a human soul ( real live intellect, will, memory = soul) . This was his human nature. But his human nature did not exist on its own, it was assumed by the Divine Natue of the Son of God ( the Second Person of the Trinity). So in Christ only one person existed, The Second Person of the Trinity. There was no human person in Christ.
If you met Jesus on the street what you would see is the Second Person of the Trinity, you would see a human body having a human soul but in and through the Divinity of Christ.
Nevertheless, Jesus’ human intellect was fully functional and active at all times even though it was operating through the Divine Second Person.
Christ’s human intellect and will were in constant communion with His Divine Intellect and Will. But it was absolutely free, yet it always obeyed and submitted itself to the Divine Will. Yet, since Christ’ had a human nature, his human intellect and will was limited and had to learn and experience what humans had to learn and experience. Fear of accidents, illness were part of what it means to have a human nature. And it was part of Christ’s sacrifice to experience all that man had to endure, except sin. And of course he was free of all concupiscence of the flesh.
So in the garden his human nature naturally feared what was about to take place. For the Divine Intellect was constantly communing with it, so he knew what was going to happen. So it was the human reaction of fear to what was going to happen that made his human nature call out to his Father. His human nature naturally knew it was united to the person of the Person of the Son of God. So the human nature cried out to the Father, who being the Father of the Second Person, was the father of Christ’s human nature as well.
More than this we can only speculate. But I also think, Christ, as God, wanted us to realize just how real Christ’s Passion was. And to impress upon us just how real Christ’s suffering and passion was he allowed Christ’s human nature cry out as any man would, expressing the real fear and horror of what was to happen. Christ wanted us to know that it was a real man suffering, that a real man had desired, in obedience, to make this sacrifice for all other men, even though he was also the Second Person of the Trinity suffering through his human nature. In this way his real love for all men could be more easily comprehended - since it was a real man suffering.