Note 1 at 2 Cor. 12:11: Proverbs 27:2 says, "Let another man praise thee, and not thine own mouth; a stranger, and not thine own lips." Paul's apostolic authority needed defending, but Paul wasn't really the one to do that. Some of the Corinthians who had received from Paul should have stood up for him. Since no one took that responsibility, Paul had to defend himself.

Note 2 at 2 Cor. 12:11: True greatness doesn't come from ourselves but from the Lord. Paul knew what he had done (not behind the very chiefest apostles-2 Cor. 11:5) but he also knew that it wasn't him that had accomplished these things (I am nothing), but Christ living through him (Gal. 2:20). True humility doesn't ignore accomplishments. It just gives the credit to the Lord (see note 18 at 1 Cor. 15:10, p. 956; see note 10 at 2 Cor. 11:5, p. 1034).