I’m really/terribly/so sorry for being late. It won’t happen again.
Please excuse my late response.
I apologise for not calling earlier.
I’d like to apologise for not returning your call.
My apologies for the delay.
Please accept my apologies for the inconvenience.
I’m afraid I can’t help you.
Excuse me for interrupting.
Sorry about the mess. I didn’t have time to clear it up.
Forgive me for what I said earlier. I didn’t mean to hurt you.
I shouldn’t have said that. Don’t be mad at me.
I was wrong. Can you forgive me?
Pardon me for being so rude.
I regret to inform you that your application has been denied. (formal)
My fault. I beg your pardon. Pardon. My bad. Sorry.