Please forgive my carelessness.
Please excuse me. I have to run.
I'm really / awfully / terribly sorry.
I cannot tell you how sorry I am.
Please forgive me. I don't mean that.
Don't worry about that.
Just forget it.
It doesn't matter at all.
It was wrong of me.
I hope you will excuse me.
I apologize for that.
...but I'm afraid I'll have to put off the meeting.
I'm really sorry to be late again.
I'm awfully sorry. I didn't realize.