I'd like to extend my genuine apology to you for having taken your music CD with me out of carelessness.
In the winter holiday, I went to England for trainings as an exchange student, and I lived in the house rented from you. But when I left, being in a hurry, I forgot to give the CD back to you and stuck it in my luggage.
To make up for my fault, I'll mail it to you as soon as possible. Once again, I sincerely apologize for the trouble and inconvenience I might have caused. I beg for your forgiveness.