Paul's use of lupeo [Greek word for "to grieve"] in 2 Corinthians suggests that he understands the Corinthians' grief as their regret and mental anxiety for their part in the rift between them and Paul.
Source: Rabbi Jonathan Kaplan - "Comfort, O Comfort, Corinth: Grief and Comfort in 2 Corinthians 7:5-13a", Harvard Theological Review, 104:4 (2011), p. 436