Can you explain Matthew 26:39?

I think your treatment of Jesus' desire is too simplistic. Jesus was determined to submit entirely to the Father's will ("as you will"). His prayer may be interpreted from different perspectives, each of which is consistent with his sinlessness.

Changing God's Mind

For example, on the surface his prayer appears to be a request that the Father change his mind, no longer requiring the crucifixion. It is not evil to want God to change his mind.

The ultimate standard for morality is not God's expressed will but his character. His expressed will is a valid standard for morality, but only as it is understood as a reflection of his character. God's words must be interpreted in order to reveal his will. We cannot simply take them at face value and assume that we have understood them. For instance, "Thou shalt not kill" is rightly understood to mean "Thou shalt not murder," and not as a prohibition against all taking of life. After all, God commanded many killings.

Also, it is within God's character to change his expressed will (e.g., Exod. 32:14; Amos 7:3,6). It is evil to disobey his expressed will, rightly understood, but it is not evil to obey it while at the same time asking him to change it. Similarly, while it is necessarily evil to desire something contrary to God's character, it is not necessarily evil to desire God to change his mind.

Your argument assumes but does not prove that Jesus' desire that God change his mind was evil. But since it is within God's character to change his mind, not all desires that God change his mind are sinful.

Complex Desires

In some sense, Jesus did not want to be punished, and this was in accordance with God's character. We are not supposed to desire to have sin imputed to us and to suffer in hell. It is not sinful to want to be saved. Or to put it another way, it is not sinful to want to be blessed by God instead of cursed by God. In fact, God wants and commands us to seek his blessings and to avoid his curses, so that Jesus' desire to avoid the curse of sin was also in accordance with God's will.

In another sense, Jesus did want to be punished. And this, too, was in accordance with both God's will and God's character. It was in accordance with God's desire because God is our savior, because he is merciful to us, and because he requires justice. It was in accordance with his will because he assigned Jesus the task of dying to atone for sin. Jesus did not want to avoid punishment unless it was "possible," that is, unless there were some other way to accomplish the atonement.

So, in wanting to avoid punishment but also determining to atone, Jesus was in perfect accordance with both the Father's will and the Father's character. Your argument confuses these senses of Jesus' will, assuming that his desire to avoid punishment was equivalent to a desire not to atone for us. It is not subtle enough to deal with Jesus' complex desires, and as a result it ends up equivocating on the definition of "want."

Meaning as Use

There is also another way to understand Jesus' prayer, one that avoids these issues. Specifically, Jesus may not have been expressing an actual request to God, and he may not have been expressing his actual desires. Instead, he may simply have been saying, "I'm scared."

Words convey meaning according to their use, and the way Jesus used his prayer was to express his fear. That is, by saying, "Not as I will, but as you will," he may well have meant, "Nevermind."

Jesus was human, and burdened with the foreboding task of bearing our sin. This was a terrifying prospect. When he prayed, he expressed his knowledge of what God had planned (God's will) and why God had planned it (God's character). He also expressed his determination to carry out the Father's plans, wanting God to change his mind only if he could find another way to accomplish the atonement ("if it is possible"). But if Jesus knew there was no other way, then taken as a whole, his prayer does not request anything at all. It simply expresses the anxiety he felt as he stood at the threshold of carrying out the Father's plan. It is not sinful to say "I'm scared."