Dear {Recipient},
I am writing you this letter today because I affected you with {crime}. I know that an apology will not be enough to make this better, but I want you to understand how sorry I am that I hurt you.
On {date}, I {crime}. I know that this has had a profound effect on your life. I have admitted my guilt and been sentenced to {number} years in prison. However, I do not feel that this punishment is making full penance for my crime.
The justice system has made sure that I have been punished, but I want you to understand that I know now how wrong what I did was. Even though there is no justification for my actions, I was blinded by {explanation}. If I could take it back, I would. Instead, please know that I will always be haunted by what I did to you. I have no greater regrets than the damage I caused you.
I know this type of communication can be stressful for victims, so I will not contact you again. However, if there is anything you would like to say to me, or if you would like to confront me, please contact me at {phone/mail}. I am willing to hear anything you have to say.
Regretfully yours,
{Sender}