Here "unceasing pain in my heart" is an idiom that Paul uses to share his emotional distress. Alternate translation: "that I grieve very greatly and deeply" (See: [[rc://en/ta/man/translate/figs-idiom]])
These two expressions mean basically the same thing. Paul uses them together to emphasize how great his emotions are. (See: [[rc://en/ta/man/translate/figs-doublet]])