According to Catholicism, what did St Paul mean by "a thorn in the flesh"?

In your question you have a scope of the Catholic Church, but sorta. If you don't mind, I have taken the liberty to add the perspective of the Latter-day Saints.

The King James Version of the verse you reference is: (The translation I am working with)

7 And lest I should be exalted above measure through the abundance of the revelations, there was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure.

8 For this thing I besought the Lord thrice, that it might depart from me.

9 And he said unto me, My grace is sufficient for thee: for my strength is made perfect in weakness. Most gladly therefore will I rather glory in my infirmities, that the power of Christ may rest upon me.

10 Therefore I take pleasure in infirmities, in reproaches, in necessities, in persecutions, in distresses for Christ’s sake: for when I am weak, then am I strong.

A "Thorn in the Flesh" is a weakness or weaknesses to remind us of our dependence on God. He recognizes why the thorn is there and is fine with his weakness so the Power of Christ can rest upon him.

The Lord seems to have several purposes in giving us trials: First, to test and prove us, to see if we are loyal to Him and to our beliefs. Second, to give us the opportunity to prove that we love Him more than any other thing. Third, to teach us something about ourselves and our commitment to remain true and focused on the eternal nature and possibilities of our lives, our families, and the mission of the Church.

When we understand the principle of opposition in all things, reasons for adversity and disappointment, and the blessings of trials, we can accept the daily events of our lives with greater hope and desire to endure well and to look unto Christ, that His power may rest upon us and see us happily through our mortal lives. Is it any wonder then that Paul glories in his infirmities or takes pleasure in reproaches, persecutions, and distress? These are among the vital means of growth and the development of true Godlike character.

Our Thorns in the Flesh, Elder Ray H. Wood, February 2003

According to Catholicism, what did St Paul mean by "a thorn in the flesh"?

7 And indeed, for fear that these surpassing revelations should make me proud, I was given a sting to distress my outward nature, an angel of Satan sent to rebuff me. - 2 Corinthians 12:7

The Church is unsure as to what St. Paul was referring too here.

There is no Early Church Father's writing, who sees his "thorn in the flesh" as referring to h*m*sexuality? Clearly both St. Paul and the Early Church writers were on the same page about this.

9 Know you not that the unjust shall not possess the kingdom of God? Do not err: neither fornicators, nor idolaters, nor adulterers, 10 Nor the effeminate, nor liers with mankind, nor thieves, nor covetous, nor drunkards, nor railers, nor extortioners, shall possess the kingdom of God. - 1 Corinthians 6:9-10

Catholic interpretations of 2 Corinthians 12:7 naturally will vary somewhat and include the following:

• One pre-Vatican II Roman Catholic writer thought that it denotes suggestions of impiety. (Easton, Matthew George (1897). "Thorn in the flesh", Easton's Bible Dictionary (New and revised ed.). T. Nelson and Sons.)

• Paul's agony over Jewish rejection of the gospel

• A reference to Paul's opponents

• A physical ailment (such as Blindness)

• The stigmata (“I have been crucified with Christ” (Gal 1:19))

Catholic interpretation continues to speculate over what this “thorn in the flesh” might mean. Some associate it with a recurrent illness or physical limitation Paul suffered. They sometimes connect it to Galatians 4:13–15. Here Paul says that he first preached to the Galatians in the “weakness” of his flesh. He commends the Galatians for welcoming him at that time; they would have plucked out their own eyes and given them to him!

St. John Chrysostom delivered an awesome homily on the subject matter:

2 Corinthians 12:7

“And that I should not be exalted overmuch, through the exceeding greatness of the revelations, there was given to me a thorn in the flesh, a messenger of Satan, to buffet me.

What do you say? He that counted not the kingdom to be anything; no, nor yet hell in respect of his longing after Christ; did he deem honor from the many to be anything, so as both to be lifted up and to need that curb continually? For he did not say, 'that he might buffet me,' but that he may buffet me. Yet who is there would say this? What then is the meaning of what is said? When we have explained what is meant at all by the thorn, and who is this messenger of Satan, then will we declare this also. There are some then who have said that he means a kind of pain in the head which was inflicted of the devil; but God forbid! For the body of Paul never could have been given over to the hands of the devil, seeing that the devil himself submitted to the same Paul at his mere bidding; and he set him laws and bounds, when he delivered over the fornicator for the destruction of the flesh, and he dared not to transgress them. What then is the meaning of what is said? An adversary is called, in the Hebrew, Satan; and in the third Book of Kings the Scripture has so termed such as were adversaries; and speaking of Solomon, says, 'In his days there was no Satan,' that is, no adversary, enemy, or opponent. 1 Kings 5:4 What he says then is this: God would not permit the Preaching to progress, in order to check our high thoughts; but permitted the adversaries to set upon us. For this indeed was enough to pluck down his high thoughts; not so that, pains in the head. And so by the messenger of Satan, he means Alexander the coppersmith, the party of Hymenæus and Philetus, all the adversaries of the word; those who contended with and fought against him, those that cast him into a prison, those that beat him, that led him away to death ; for they did Satan's business. As then he calls those Jews children of the devil, who were imitating his deeds, so also he calls a messenger of Satan every one that opposes. He says therefore, There was given to me a thorn to buffet me; not as if God puts arms into such men's hands, God forbid! not that He does chastise or punish, but for the time allows and permits them. - Homily 26 on Second Corinthians

The Catholic Haydock Commentary on 2 Cor. 12:7 says:

A sting of my flesh,[1] an angel, or a messenger of Satan, to buffet me. The Latin word [stimulus] signifies any thing that pricks or stings, the Greek word [σκόλοψ] a sharp stick or pale: he speaks by a metaphor, as also when he says to buffet me; that is, by causing great trouble or pain. Some understand by it a violent headache or pain, or distemper in the body. St. Augustine mentions this opinion, and does not reject it, in Psalm xcviii. tom. 4. p. 1069.; in Psalm cxxx. p. 1465. St. Jerome also speaks of it in chap. iv. ad Galatas, tom. 4. p. 274, Ed. Ben. But St. Chrysostom, by the sting, and the angel of Satan, understands that opposition which St. Paul met with from his enemies, and those of the gospel; as Satan signifies an adversary. Others understand troublesome temptations of the flesh, immodest thoughts, and representations, suggested by the devil, and permitted by Almighty God for his greater good.