We don't know what Paul's "thorn in the flesh" was because he doesn't tell us. Whatever it was, it was a chronic & debilitaling physical problem, which at times kept him from working. This thorn was a hindrance to his ministry, & he prayed for its removal; but God refused. Paul was a very self-sufficient person, so this thorn was difficult for him. It kept Paul humble, reminded him of his need for constant contact with God, and benefited those around him as they saw God at work in his life.
Although God did not remove Paul's physical affliction, he promised to demonstrate his power in Paul. The fact that God's power shows up in weak people should give us courage. We must rely on God for our effectiveness rather than on simple energy, effort, or talent. Our weakness not only helps develop Christian character; it also deepens our worship, for in admitting our weakness, we affirm God's strength.
God does not intend for us to seek to be weak, passive, or ineffective - life provides enough hindrances & setbacks without us creating them. When they come, we must depend on God.
Paul was not merely revealing his feelings; he was defending his authoring as an apostle of Jesus Christ. He was hurt that the church in Corinth doubted & questioned him, & he defended himself for the cause of the Gospel, not to satisfy his ego.
Reference summary used from the Life Application KJV, Tyndale House Publishers, Inc., Wheaton, Illinois