And he went a little farther, and fell on his face, and prayed, saying, O my Father, if it be possible, let this cup pass from me: nevertheless not as I will, but as thou wilt. (Matthew 26:39)
How great was the sorrow of Jesus Christ! This grief was spoken of throughout the Psalms:
my heart is like wax; it is melted in the midst of my bowels.(Psalm 22:14)
The sorrows of death compassed me, and the floods of ungodly men made me afraid.The sorrows of hell compassed me about: the snares of death prevented me. (Psalm 18:4-5)
My heart is sore pained within me: and the terrors of death are fallen upon me.Fearfulness and trembling are come upon me, and horror hath overwhelmed me.(Psalm 55:4-5)
Indeed, Isaiah prophesied of him that he would be “a man of sorrows, and acquainted with grief” (Isaiah 53:3).
What was the cause of his sorrow? Some have mistakenly thought that Christ was struggling with the Father, as if his will was at odds with the will of God. This is not the case. The cause of his sorrow was that the weight of man’s sin was being laid upon him. Indeed, the awareness of the just penalty for sin was being manifested in his person. As Matthew Henry comments,
He knew the malignity of the sins that were laid upon him, how provoking to God, how ruining to man; and charged upon him, he was sorrowful and very heavy. Not it was that iniquities took hold on him; so that he was not able to look up, as was foretold concerning him (Ps.40:7,12)
Christ was bearing the burden for our sins. This was the cause for his sorrow. Unlike the martyrdom of those in Christ, who bear their suffering joyfully since a promise is annexed to it (Matthew 5:10,12), Christ bore his suffering and death for a curse. Certainly, he endured the cross for the joy that was set before him (Hebrews 12:2). However, that joy came only after he bore the curse that we justly deserved. Here, it is the curse which is upon him and weighing down his soul.
And yet, through all of this, we see that Christ’s human will was in conformity with the divine will, thus demonstrating the presence of two wills rather than their absence; for he says “nevertheless not as I will, but as thou wilt” (v.39).
Calvin’s commentary is so helpful here that I must quote it in its entirety. He explains the two wills of Christ – human and divine – and how this passage of Scripture may be practically applied by us in our moments of trial and temptation. When Christ prayed in the garden, his will was never – at any time – contrary to the will of the Father. This scene demonstrates without a shadow of a doubt that his human will was in conformity with his divine will which is one with the Father. May we, in our trials and temptations, endure them with the same spirit, trusting that His will is right and good.
Christ, amidst the utmost vehemence of grief or fear, restrained himself within proper bounds. Nay more, as musical sounds, though various and differing from each other, are so far from being discordant, that they produce sweet melody and fine harmony; so in Christ there was a remarkable example of adaptation between the two wills, the will of God and the will of man, so that they differed from each other without any conflict or opposition.
This passage shows plainly enough the gross folly of those ancient heretics, who were called Monothelites,  because they imagined that the will of Christ was but one and simple; for Christ, as he was God, willed nothing different from the Father; and therefore it follows, that his human soul had affections distinct from the secret purpose of God. But if even Christ was under the necessity of holding his will captive, in order to subject it to the government of God, though it was properly regulated, how carefully ought we to repress the violence of our feelings, which are always inconsiderate, and rash, and full of rebellion? And though the Spirit of God governs us, so that we wish nothing but what is agreeable to reason, still we owe to God such obedience as to endure patiently that our wishes should not be granted;  For the modesty of faith consists in permitting God to appoint differently from what we desire. Above all, when we have no certain and special promise, we ought to abide by this rule, not to ask any thing but on the condition that God shall fulfill what he has decreed; which cannot be done, unless we give up our wishes to his disposal.
Endure the sufferings of this life in a manner like Christ, humbling yourself before the will of God, saying and believing “nevertheless not as I will, but as thou wilt.”