I was just reading, on the recommendation of my priest, the Rainbow Series by Fr. Thomas Hopko. In (I believe) Vol. I, I - for the first time in my life, that I can recall - really understood (within the confines of a human mind) the Trinity.
Divinity is the essence of God the Father, the Son, and the Holy Spirit. As such, when you look at any one of them, you can ask "What is that?" and the answer is "That is God." But, if you ask "WHO is that?" then the answer is not "God" unless you mean the Father. The answer is either "It is the Father" or "It is the Son" or "It is the Holy Spirit." They are truly each separate persons. Fr. Hopko, in fact, said it is much like the following analogy (which I am going to add words to to belabor the point, and tie it in more directly with the idea of Christ praying):
If you look at a person on the street and ask what that is, the answer is obviously that it is a human being. That is the what; that is the essence. You would not assume that his being a human being means he is the same person as you, who are also a human being. If you see that same man talking on the phone, after you note that the what he is, is a human being, you would not assume that he is talking to himself merely because you know the person on the other end is also a human being. Rather, you would then ask yourself WHO the man on the street is and WHO the person on the other end of the phone is.
So, to wrap up my disorganized post, the Father, the Son, and the Holy Spirit are all God because they all share equally in the same divinity, the divinity which is their essence; the divinity which defines what it means to be God. However, that is only the what and not the who. Who Christ is, who the Father is, and who the Holy Spirit is, is not divinity. Christ is not divinity. Rather, He is the Son of the Father.