I am reading Barry D Smith's Paul's Seven Explanations of the Suffering of the Righteous (New York: Peter Lang, 2002). Here are some good quotes about 2 Corinthians.
Paul would argue that he is not naturally weak; rather, God has made him weak by means of this "thorn in the flesh." (p 167)
In his view, God intentionally humiliated him in this way in order to give him a much-needed sense of the need of reliance on another power. (p 168)
The confession of his inherent incapacity is a pre-condition of the ingression of the power of Christ. The pedagogical effect of his "thorn in the flesh," therefore, is invaluable to Paul's apostolic ministry; his affliction prevents his entering into a state of illusory and unfruitful self-confidence, which would cut him off from the power of Christ. (pp 168-9)
Nothing succeeds like success in breeding self-confidence; self-confidence would not be self-confidence unless the self-confident one is convinced of the sufficiency of his innate abilities. According to Paul, however, there is no place in the apostolic ministry for self-confidence for reasons already stated. So when Paul experiences success in his ministry as the apostle to the gentiles, God strategically forestalls the natural movement to self-confidence occasioned by his success by enforced weakness, as well as by "insults, hardships, persecutions and difficulties." (p 169)
Thus, suffering functions to counterbalance the effects of success by reminding Paul that his success has not come from himself, but only through the power of Christ. As a result, he is in a position to appropriate that other power. This is the pedagogical benefit of suffering. (pp 169-70)
The apostles seem always to be on the verge of being overwhelmed by the suffering resultant from their commonness and fragility, and thereby of being rendered ineffectual, but are never actually overcome. It is this model of apostleship that Paul's residual opposition Corinthians find objectionable. they cannot fathom how men who are constantly teetering on the brink of personal collapse can be genuine apostles.
All human beings offer nothing to God. In Paul's view, the realization that this is the human condition is the single qualification for being an apostle. (p 174)