I am so glad that you asked that!
2 Cor 12
7. To keep me from becoming conceited because of these surpassingly great revelations, there was given me a thorn in my flesh, a messenger of Satan, to torment me.
8. Three times I pleaded with the Lord to take it away from me.
9. But he said to me, "My grace is sufficient for you, for my power is made perfect in weakness." Therefore I will boast all the more gladly about my weaknesses, so that Christ's power may rest on me.
We are not sure what Pauls particular "thorn" was but we do know that he had something that constantly bugged him. Some say it was lust but I eshew that because he didn't really from a hermanutical perspective seem to have a problem there. However guilt for his former life of persecuting Christians does come into play since he mentions that "He was greatest among sinners" many times. And from empirical anyalisis of his writings depression may be his yoke as we can see wide variances in his timber when speaking. This subtely indicitive of what they call today "manic depression"