Apology for the Thoughtlessness:
12, XYZ Road,
New York,
USA
24th May, 200*

Dear Peter
I am writing this letter to apologize to you formally or my thoughtlessness. It was immature and foolish of me to talk about you and your business with someone else, in such a reckless manner, though unintentionally. I am extremely sorry for such a foolish act. I realize, I’ve hurt your fallings and hope you will forgive me as it was actually an inadvertent mistake. I assure and promise you that I will never let this happen again.

With regards,

Sincerely
John