Dear Bob, I am writing to you today to apologize for my mistake and to express the regret and sorrow that I feel. I know that I have hurt you and I am truly sorry. I want you to know that I love you and I always have. I regret the situation that has occurred and I understand why you are hurt and angry. I should have been honest with you and I am sorry that I was not. I understand that I have made a mistake and I want to improve as a person and make sure it does not happen again. I will work hard to regain your trust and prove to you that I am committed to our relationship. I know that these words cannot take away the pain I have caused, but I hope that you can find it in your heart to forgive me. Sincerely, [Your Name]