Dear {Recipient},
I am writing to apologize for my absence during {event}. I know you expected me to be there, and it was important that I attend, but I was simply unable to make it.
I hope you won't see this as a reflection of who I am as a whole. Ordinarily, when I make a promise, I am able to keep it, so I am both sorry and embarrassed that I was not able to be true to my word in this case.
Please call me as soon as you can. We can discuss the matter, and I can begin to make amends for my inability to follow through.
Sincerely,
{Sender}
