What do you believe is the old man?
The man born from Adam. The Adamic creature of the Adamic creation.
And the new man is the man born from God.
You don't seem to be associating it at all with our nature. What else do you think it is?
Do you realize "Nature" isn't used in the Bible the way it is so often used by us? We talk about a "sin nature", for instance, but the Bible does not.
Let me say that again, the Bible does not speak of a sin nature. What is speaks of is corrupt flesh.
Our "nature" just describes how we are. The "nature" of a rock is that it is hard, dense, unliving. Things have a "nature", but that's just a description of how and what they are. The fleshy man born from Adam's line has it's nature, as we'd call it, but the Bible just calls it corrupt.
The spiritual man born from God has it's nature, the Bible just calls it righteous. The Bible doesn't talk about a thing's nature, except in 1 Corinthians 15, in discussing the earthy and celestial, but still not in the terms that people generally do on this topic.
We were a corrupted being born from a corrupted humanity. Being born from God, we are now an uncorrupted being born from an incorruptible God. And we inhabit that same body, that remains corrupted, until it is transformed in our resurrection.
The body is just a shell, the puppet that the mind and emotions orchestrates.
The body has it's own brain, which is just as corrupt as the rest of the body.
Romans 8:5-14 KJV
5) For they that are after the flesh do mind the things of the flesh; but they that are after the Spirit the things of the Spirit.
6) For to be carnally minded is death; but to be spiritually minded is life and peace.
7) Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be.
8) So then they that are in the flesh cannot please God.
9) But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.
10) And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness.
11) But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you.
12) Therefore, brethren, we are debtors, not to the flesh, to live after the flesh.
13) For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live.
14) For as many as are led by the Spirit of God, they are the sons of God.
The "carnal mind", that is, the "fleshy mind". It doesn't exist apart from the brain. It's a product of our personhood housed in a electro-chemical brain. And in the case of mental illness, that brain is damaged, and the mind is likewise damaged, being a product of the brain.
But we have the mind of Christ - NOT a product of our own brains, but this new mind, our whole new life, comes from the Spirit of God. Mental illness does not affect the spiritual mind.
We are to put off the old man - the man born from Adam, corrupt - and put on the new man - the man born from God, righteous. The old man, who we were before Christ. The new man, who we are in Christ.
Stop living as though unregenerate, in the old ways, following the old patterns of the mind of the flesh. Start living a alive from the dead, following Christ.
Much love!