I trust you will pardon me for——
I write to apologize for my failure to——
I hope you will forgive me when I tell you that——
I am sorry I could not do as I had promised to do
I am very much mortified to find that——
Trusting you will forgive my great carelessness
I hope you have not suffered any inconvenience from——
Hoping you will pardon the——
I am afraid you will accuse me of rudeness, but I——