What letter is Paul talking about in 2 Corinthians 7:8?

Paul is referring to his first letter to the Corinthians—the first Corinthian letter. He had to address their problems and make them sorry, but he did not regret or repent of sending it to them because that correction was necessary.

Les explains that Paul was at Ephesus while Titus was at Corinth. Paul had written a letter to the Corinthians in which he had to dress them down severely. He had heard reports from Corinth that were against him and condemned him, but Titus brought good news that the Corinthians still loved Paul and held him in high esteem, so Paul rejoiced. In verse 8, Paul says, "though I made you sorry with a letter." Les identifies that letter as the first Corinthian letter, the one in which Paul addressed their problems. Paul says he does not repent or regret sending it, because the correction had to be done.

2 Corinthians 7:8

For though I made you sorry with a letter, I do not repent, though I did repent: for I perceive that the same epistle hath made you sorry, though it were but for a season.

Did Paul regret causing the Corinthians sorrow with his letter?

Paul did not regret causing the Corinthians sorrow by his first letter, even though it grieved them. He says he does not repent—better understood as regret—because the correction had to be done. The sorrow was necessary for addressing their problems, and Titus later brought good news of their affection for him.