The following is a conversation from an English textbook (Summit 2A by Joan Saslow and Allen Ascher; Pearson Education, 2012) in which a person expresses regret about breaking his friend's camera.
Tim, I hate to tell you this but I dropped the camera you lent me, and it can't be fixed.
Oh, no. How did that happen?
Well, I tripped, and it fell out of my bag. I feel awful about it.
Are you sure it can't be fixed?
Pretty sure. I took it to the camera shop, and they said to forget it. But I can replace it with a newer model.
Then the book suggests other ways to express regret as the following: I'm so sorry; I feel awful (about it); I feel (just) terrible (about it).
The problem is it says I'm so sorry does not conclude with about it. Really? I'm pretty sure I've heard sorry about something or someone. What am getting wrong here?! Is it something about so before sorry?