Except Paul wasn't talking about losing his salvation. He was talking about being a testimony to others through the life he led and that if he let his old nature prevail and fell into sin, others would not hear his message, his preaching would be rejected.
I myself should be a castaway, or rejected, or disapproved of; that is, by men: the apostle's concern is, lest he should do anything that might bring a reproach on the Gospel; lest some corruption of his nature or other should break out, and thereby his ministry be justly blamed, and be brought under contempt; and so he be rejected and disapproved of by men, and become useless as a preacher: (Gill's Exposition of the Bible)
Paul is not merely referring to "some corruption of his nature or other breaking out, and of his ministry being blamed, and be brought under contempt; and so he be rejected and disapproved of by men," but of being shipwrecked, reprobate - which is how the word for castaway is usually translated - due to not keeping his body in subjection.
While this need not mean he would die as a reprobate, elsewhere Paul states, 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. (Romans 8:13)
And warns the liberated Galatians believers against being entangled again with the yoke of bondage, and in which case Christ would profit them nothing, becoming "of no effect," as being "fallen from grace." (Galatians 5:1-4)
Likewise God warns believers by the writer of Hebrews to "Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God," (Hebrews 3:12) and of sinning wilfully after they had believed the gospel, drawinh back unto perdition, versus believing to the saving of the soul. (Hebrews 10:25,38,39)
Therefore God works to chasten us when we go astray, that we should not be condemned with the world. (1 Corinthians 11:32)
Thanks be to God.
It doesn't say anything about his preaching being rejected, but about he, himself, being "disqualified".