I've insulted one of my dearest friends
How do you feel about having insulted your friend?
I feel pretty bad about it. But she is so thick-skinned
I am glad you are having such remorse and regret over you nasty actions. No friend deserves to be insulted and disrespected, and instead of feeling sorry for yourself why don't you own up to her too and apologize?
My friend don't take insults seriously, plus she has PMS, so she's angry at everyone, even her husband and children. I bet she was stressed out, which is why she acted like a bitch.
Even if she acts like she's not taking it seriously it's not nice to insult her. You should try to understand if she's stressed out and think about her feelings even if she doesn't seem upset about the insults.