I don't think the thorn in the flesh is a particular sin struggle ... I think it's exactly what Paul says it is: a messenger from Satan. And yes I believe it can be there from childhood. I've known people who I believe have this issue. It's horrific. This is why I don't think it's a struggle with "sin". It is something else. How this spirit manifested with Paul, has been debated. Was it through a person who hounded his ministry ? Was it a direct attack on him ?
A key to understanding it though, I think, is the way Paul describes it:
It buffets him, torments him, and it is a thorn in the FLESH. In other words, it effects his flesh. And also, it's a messenger of Satan … a separate entity, separate from himself. It's not the desire to sin that is causing Paul an issue … it is like being stabbed with a sharp object. Buffeted.
Envision someone being tormented with something physical. As though you walked into a room and suddenly there was a foe, and this foe came up on you, and started to stab you with a small knife, in different parts of your body, and it wouldn't stop. You have the authority of Christ with you, but this spirit won't leave or honor it. So your authority doesn't work, and there is a reason for it. God showed Paul this reason for him personally.
In the examples I am thinking of, I've known people who have experienced their body being abused, beaten, etc, by a spirit. In the lighter forms, it's as though their senses are attacked … sight, sound, the nerves in their body. Like being smashed in the head with bricks. In movies, this is a possessed person. But that's the movies. If it's someone who has the Spirit of Christ, then it is a confusing reality, and not one easily understood by others who haven't walked in those same shoes. It is similar to trying to understand why the apostles and others have suffered at the hands of others at any time, especially if they were doing the will of God. So if your body is experiencing this … as though you are actually being beaten, abused, tortured, tormented … PHYSICALLY … then I think you may be dealing with such a messenger of Satan. So it's not an "internal struggle". It is as though a person is standing there hitting you, abusing you, in the flesh, tormenting it. It's not just the "internal struggle", it manifests physically on it's own. These people (that I've known at least) have experienced, at times, physical marks on their body, burns, strange rashes, bruising, and their body reacts strongly. Sights, sounds, smells, as though being slammed in the head over and over.
And I've found that trying to rationalize reasons as to why God would allow it, doesn't cut the mustard ... it only infuriates and disillusions a person more because trying to rationalize evil that is happening to you doesn't usually bring forth much but denial and/or disillusionment. I've found it helpful to find out the exact reason from God directly for why a thorn would continue to be there, and if it is such a thing in the first place. God told Paul His grace was sufficient for him ... and perhaps He showed Paul more as well on the issue (like Paul's tendency to be conceited, which I could see perhaps giving Paul a "god complex" or sending him right back to his murderous ways). In one particular instance I'm thinking of with someone I know, they believe God showed them why it was there, and after YEARS of dealing with it, they understood more why it hadn't been removed yet. But that was after years, and God showed them directly why it hadn't been removed.
My understanding of this is backed by direct experiences that parallel Paul's account, not Biblical exegesis … so I'm not arguing it from a scholastic standpoint. I'm not debating exactly how Paul experienced this or whatever, only that I'm seeing the direct parallels with those I've known and it fits with Paul's own words in that instance in a lot of ways.
Again I think the key, though … apart from God showing you directly that it is … is you are able to point to a separate entity. Not just yourself. Not just the internal burn and struggle or whatever. And your body reacts to this entity abusing it, tormenting it, in a variety of ways. But it's an ENTITY. It's not a desire.
