Sorrow is not bad if it brings someone to a place of repentance. At the time that Paul wrote the letter of tears he felt bad, however, now having seen the effect it had he is not sorry because his pain induce sorrow in the Corinthians that led them to repentance and ultimately sending Titus to contact Paul and bring him encouragement at the time he needed it. This is the importance of accountability and my wife is much better at this than I am, though I am trying to get better. Paul did not mask his hurt from the Corinthians, he poured it out, but he poured it out in such a way as to convey his pain and as a result he was ultimately comforted. What if Paul kept his hurt to himself. What if he never "confronted" the Corinthians about their behavior. They may have never repented and Paul would not have received comfort from them at a time when he needed it most. Don't be afraid to have hard conversations. They make things and Us better.