2 Corinthians 7 "Paul's Joy"

This chapter is mainly about how Paul is overjoyed when he had met up with Titus.  They talked about the town of Corinth and how Paul's first letter had made them sad and how when Paul was there he was being given a hard time, but then Titus told Paul how they turned away from there sin and how the sadness they had was not worldly it was sadness with God which is explained to come from guilt of sin and is what brings us back to God. Paul who expresses he and his followers never wanted to make them sad is not upset for sending the letter but joyful and is happy the faith he always had in the Corinthians is complete.  

What gets me is that I have been fooled a couple of times, or atleast I thought I was, when I chose to call someone out on their sin.  How does Paul know when he can and can't say something(send a letter, or visit them).  I feel the risk of turning someone away from God has kept me from doing his will atleast once, I don't know when but I know it has happened.