Dear {Receiver}
I’m sorry. I never wanted—or expected—this to happen, but I guess things don’t always go as hoped.
There’s no easy way to say this, but we have gone as far as we could go in our marriage.
We’ve had some great times, but we would be doing each other great distress by ignoring our less glamorous moments.
Enclosed are the divorce papers that need to be signed. If you have any questions, please contact my attorney,{attorney name}.
Sincerely,
{Sender}
{Sender}