I am not sure if mental illness is a fair thing to say for every one, if at all.
What was happening to Paul, he seems to be ascribing what we call mental illness as an infirmity.
2 Corinthians 12:5 Of such an one will I glory: yet of myself I will not glory, but in mine infirmities. 6 For though I would desire to glory, I shall not be a fool; for I will say the truth: but now I forbear, lest any man should think of me above that which he seeth me to be, or that he heareth of me. 7 And lest I should be exalted above measure through the abundance of the revelations, there was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure. 8 For this thing I besought the Lord thrice, that it might depart from me. 9 And he said unto me, My grace is sufficient for thee: for my strength is made perfect in weakness. Most gladly therefore will I rather glory in my infirmities, that the power of Christ may rest upon me. 10 Therefore I take pleasure in infirmities, in reproaches, in necessities, in persecutions, in distresses for Christ's sake: for when I am weak, then am I strong.
That thorn in the flesh is the messenger of Satan that is buffeting Paul. That sounds to me more like hearing voices accusing him.
I know that the popular train of thought is that his thorn in the flesh was his poor eyesight, and they can go to another part of scripture to show it by his large handwriting, but that is hardly symbolic of a messenger of Satan buffeting him in the flesh.
And what is more, I doubt having failing eyesight demotes him in the eyes of people that would exalt him for the abundance of revelations Paul had been giving from all his epistles to the churches. Hearing voices does.
Today, if Paul was alive, would his failing eyesight keep people from exalting him too much? I don't think so.
If Paul gloried in his infirmity that he was hearing voices accusing him, I think that would keep people from exalting Paul period. That way, Paul is decreasing in the eyes of the people so that Christ is increasing.
John 3:28 Ye yourselves bear me witness, that I said, I am not the Christ, but that I am sent before him. 29 He that hath the bride is the bridegroom: but the friend of the bridegroom, which standeth and heareth him, rejoiceth greatly because of the bridegroom's voice: this my joy therefore is fulfilled. 30 He must increase, but I must decrease.
It is what Paul has said and written in regards to His words that is to be confirmed by Him that is where the believers should have the spotlight on; not on Paul; not on the Holy Spirit; but on the Son of God; the Bridegroom where the abiding bride of Christ should be looking to always by His grace and by His help in living that reconciled relationship with God through Jesus Christ.