Boyfriend yelled at me for sending his ex a message?

I saw that she put his last name down on facebook so I messaged her and said....oh so now you are using his last name? forget about it. he's never going back to you........then she wrote me back calling me a bed wench and she texted him and said.....please tell her to leave me and the kids alone. I'm done with you and her......so he cussed me out and broke up with me for that. I didn't think it was a big deal. he told me he wants a big apology today from me. what should I do? how can I fix it?