Dear [Parent's Name],
I'm writing this letter to apologize for the way I've been acting lately. I know I've been getting smart, talking back, and not listening, and I want you to know that I am truly sorry. I realize that my behavior has been disrespectful and hurtful, and I promise that I will make a conscious effort to change.
You are the best parent anyone could ask for, and you don't deserve to be treated in such a way. I appreciate all that you do for me, and I want you to know that I am grateful for your love and support. I will do my best to be a better son/daughter from now on and show you the respect that you deserve.
Thank you for always being there for me and for your endless patience and understanding. I love you.
Sincerely,
[Your Name]