Here is the one Dr. Harley had in SAA:
My Dear Sue,
I apologize to you for my part in creating an environment that helped make your affair with Greg possible. I foolishly pursued my career without understanding my responsibility to meet your most important emotional needs. I was not there for you when you needed me most, and we are now both suffering for my mistake.
I am willing to avoid the mistakes I've made in the past and create a new life for both of us that meet your needs. But I cannot do that until you end your relationship with Greg once and for all.
Until then, I will avoid seeing you or talking to you. I will also not be able to help you financially. Our friends Jane and Paul have agreed to help make arrangements for you to visit the children whenever you would like. But I will not be here when you visit. If you want to communicate about the children or any other matter, it will have to be through Jane and Paul.
I ask you to respect my decision to separate from you this way. You must know about the suffering I have endured because of your relationship with Greg, and I simply cannot be with you any longer, knowing that you are with him. I still love you but I cannot see you under these conditions.
As soon as you are willing to permanently separate from Greg and are willing to follow the measures that were suggested to ensure total separation, I will be willing to discuss our future together.
I want us to be able to rebuild our marriage someday. I want us to be able to meet each other's emotional needs and to avoid doing anything to hurt each other. We need to build a new lifestyle in which everything we do makes us both happy. Then there will never again be a reason for us to separate. I want to be your best friend, someone who is always there for you when you need me. And I want you as my best friend.
I loved you when we married and I continue to love you right up to this day. I just cannot be with you or help you as long as you are seeing Greg.
With my love,
Jon
I used a version of this when I wrote my Plan B letter. I added a lot of personal things to make it "mine". I reminded my H of all of our good times, vacations, holidays, etc.