My thoughts on Paul's thorn in the flesh was not physical at all. I think he suffered from the same thing many of us Christians suffer from today. The memory of past sins and failures continuously haunting us. A messenger from Satan telling us over and over that we are not worthy of God's grace. Many Christian including Paul are haunted by their past. Paul watched as Stephen was stoned to death. What a horrible memory that would be to keep popping up all the time. I'm sure Paul had many more excruciating memories.