This is a sample apology letter for the Thoughtlessness.
Dear Mr. ………………………… (Person Name)
I am writing this letter to apologize to you formally for 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 feelings 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.