/u/PluniaZ's answer is probably a bit dense to understand, but Aquinas cite's St. John of Damascus's De Fide Orthodox in that passage which is really helpful here.
Here is the passage from the work relevant:
Prayer is an uprising of the mind to God or a petitioning of God for what is fitting. How then did it happen that our Lord offered up prayer in the case of Lazarus, and at the hour of His passion? For His holy mind was in no need either of any uprising towards God, since it had been once and for all united in subsistence with the God Word, or of any petitioning of God. For Christ is one. But it was because He appropriated to Himself our personality and took our impress on Himself, and became an ensample for us, and taught us to ask of God and strain towards Him, and guided us through His own holy mind in the way that leads up to God. For just as He endured the passion, achieving for our sakes a triumph over it, so also He offered up prayer, guiding us, as I said, in the way that leads up to God, and "fulfilling all righteousness" on our behalf
when he said, Father, if it be possible, let this cup pass from Me: yet, not as I will but as Thou wilt, is it not clear to all that He said this as a lesson to us to ask help in our trials only from God, and to prefer God's will to our own, and as a proof that He did actually appropriate to Himself the attributes of our nature, and that He did in truth possess two wills, natural, indeed, and corresponding with His natures but yet in no wise opposed to one another? "Father" implies that He is of the same essence, but "if it be possible" does not mean that He was in ignorance (for what is impossible to God?), but serves to teach us to prefer God's will to our own. For that alone is impossible which is against God's will and permission. "But not as I will but as Thou wilt," for inasmuch as He is God, He is identical with the Father, while inasmuch as He is man, He manifests the natural will of mankind. For it is this that naturally seeks escape from death.