1. I'm terribly sorry.
2. Sorry. It was all my fault.
3. Please don't be mad at me.
4. Please forgive me.
5. I'm sorry. I didn't mean to offend you.
6. How can I make it up to you?
7. It's my fault. I really feel bad about it.
8. I just don't know what to say.
9. I shouldn't have done that.
10. It was so stupid of me. I really do apologize.
11. I'm sorry, I spoke out of turn.
12. It's my mistake. It won't happen again.