I am perplexed.
Everyone seems to be debating what Paul's thorn in the flesh was when it states quite clearly it was a messenger of Satan.
In other words it was a demon, not a thing, not a symptom, but a personality of evil from the enemy, and it attacked his flesh, his physical frame.
The various previous answers above are all the possible results of that demon's attack, not the thorn/demon itself.
The one thing completely missed though is the Old Testament connection.
Paul was an arch theologian and never missed an opportunity to cross refer if you are observant,
I believe he was referring to several OT passages, notable Judges2v3
3Therefore I also said, ‘I will not drive them out before you; but they shall be
thorns in your side, and their gods shall
[c]be a snare to you.’ ”
Josh23v13 know for certain that the LORD your God will no longer drive out these nations from before you. But they shall be snares and traps to you, and scourges on your sides and thorns in your eyes, until you perish from this good land which the LORD your God has given you.
NKJV
55But if you do not drive out the inhabitants of the land from before you, then it shall be that those whom you let remain
shall be irritants in your eyes and thorns in your sides, and they shall harass you in the land where you dwell.
I do not see how we can ignore the OT link, typical of Paul.
So what does it mean?
8Concerning this thing I pleaded with the Lord three times that it might depart from me.
9And He said to me, “My grace is sufficient for you, for My strength is made perfect in weakness.” Therefore most gladly I will rather boast in my infirmities, that the power of Christ may rest upon me.
Three times Paul cried out to the Lord to remove it, but as we know from the OT, it was the duty of the Israelites, empowered by the Lord, to remove the enemies of Israel, and the minute they put their feet up, the Lord ceased helping them.
Translated for Paul's Thorn, it was Paul himself that had to pull that thorn out, (deliverance) under the grace, or empowerment of the Holy Spirit.
Likewise it is we who must drive out the inhabitants of our land. We cannot ask God to do what He has commanded us to do.