The thorn for the flesh was needed when Paul came down from paradise. It was not that the flesh had improved. The thorn was something to hinder sin, something that made the Apostle outwardly contemptible in his ministry. Each believer probably has a different thorn according to his need. The flesh never changes, but the power of the Spirit of God is such that the flesh is kept down. "Always bearing about in the body the dying of the Lord Jesus" would not be necessary if the flesh could improve. We should so walk through the wilderness with our flesh in the place of death, as it were.
God looks at us as dead with Christ, and we are called on to reckon ourselves dead. Our title to this claim is that Christ has died, and we are crucified with Him. It is not only that we are born of God, but we have died with Christ.
In Colossians 3 you find God seeing us as dead. In Romans 6 I reckon myself dead. In 2 Corinthians 4 it is "always bearing about in the body the dying of the Lord Jesus, that the life also of Jesus might be made manifest in our body." This is going very far indeed. Death to Paul was so realized that the life of Christ only worked in him.