Q&A for How to End a Letter of Apology

Try something like, "Once again, I apologize for my absence. I will try to be better in the future." Then sign your name.

If you don't know the person's address, have someone else deliver the letter.

Yes. A signature authenticates and formalizes the apology.

You certainly can! "Sincerely" or "Apologetically" would also work just as well as sign-offs before a signature, but you can write "Sincerely apologetic" if you want to.