Jamieson, Fausset and Brown on 2 Corinthians 7:8

8. with a letter--Greek, "in the letter" namely, the first Epistle to the Corinthians.

I do not repent, though I did repent--Translate, "I do not regret it, though I did regret it." The Greek words for regret and repent are distinct. Paul was almost regretting, through parental tenderness, his having used rebukes calculated to grieve the Corinthians; but now that he has learned from Titus the salutary effect produced on them, he no longer regrets it.

for I perceive, &c.--This is explanatory of "I did repent" or "regret it," and is parenthetical ("for I perceive that that Epistle did make you sorry, though it was but for a season").