How do you interpret (in bold), "But we have this treasure in clay vessels, that the exceeding greatness of the power may be of God, and not from ourselves"? You need to take it in the context of the whole of the preceding chapter (chapters and verses were invented hundreds of years after the Scriptures were completed, don't forget), for it's all part of one speech/subject. At the very least start from verse 7 of the previous chapter:
(2Co 3:7) But if the service of death, written engraved on stones, came with glory, so that the children of Israel could not look steadfastly on the face of Moses for the glory of his face, which was passing away;
(2Co 3:8) won’t service of the Spirit be with much more glory?
(2Co 3:9) For if the service of condemnation has glory, the service of righteousness exceeds much more in glory.
(2Co 3:10) For most certainly that which has been made glorious has not been made glorious in this respect, by reason of the glory that surpasses.
(2Co 3:11) For if that which passes away was with glory, much more that which remains is in glory.
(2Co 3:12) Having therefore such a hope, we use great boldness of speech,
(2Co 3:13) and not as Moses, who put a veil on his face, that the children of Israel wouldn’t look steadfastly on the end of that which was passing away.
(2Co 3:14) But their minds were hardened, for until this very day at the reading of the old covenant the same veil remains, because in Christ it passes away.
(2Co 3:15) But to this day, when Moses is read, a veil lies on their heart.
(2Co 3:16) But whenever one turns to the Lord, the veil is taken away.
(2Co 3:17) Now the Lord is the Spirit and where the Spirit of the Lord is, there is liberty.
(2Co 3:18) But we all, with unveiled face seeing the glory of the Lord as in a mirror, are transformed into the same image from glory to glory, even as from the Lord, the Spirit.
(2Co 4:1) Therefore seeing we have this ministry, even as we obtained mercy, we don’t faint.
(2Co 4:2) But we have renounced the hidden things of shame, not walking in craftiness, nor handling the word of God deceitfully; but by the manifestation of the truth commending ourselves to every man’s conscience in the sight of God.
(2Co 4:3) Even if our Good News is veiled, it is veiled in those who are dying;
(2Co 4:4) in whom the god of this world has blinded the minds of the unbelieving, that the light of the Good News of the glory of Christ, who is the image of God, should not dawn on them.
(2Co 4:5) For we don’t preach ourselves, but Christ Jesus as Lord, and ourselves as your servants for Jesus’ sake;
(2Co 4:6) seeing it is God who said, “Light will shine out of darkness,” who has shone in our hearts, to give the light of the knowledge of the glory of God in the face of Jesus Christ.
(2Co 4:7) But we have this treasure in clay vessels, that the exceeding greatness of the power may be of God, and not from ourselves.
So we are transformed into the same image of the glory of the Lord - as it shines in his face so it would shine in our (new creature's) face. But it is hidden in a clay vessel (human body) so that the greatness of the power of the Gospel may be seen to be of God and not from us.