Here it is. Any advice is welcome.

Dear WS,

I am writing this letter to you and it will be the last one. I have sent many emails and talked to you about our situation but it just not helped. Despite the hurtful things we have said and did over the last 6 months, I still love you very deeply. I love you more than you will ever know.

I want you to remember me as the man you fell in love with and not the person I became during this ordeal. My goal is to take care of myself and go back to who I was because I liked that person. I was energetic, upbeat, kind and always optimistic.

At this point, it is best that we do not contact each other than for the sake of the children. It gives me false hope, which turns into anger and resentment. I did not sleep for 2 nights after I seen you Sunday night which is why I called so angry Tuesday morning. I will never forgive myself for the horrible things I said to you that day.

I am still here for you but it can only be for reconciliation if you chose to do so. I have learned so much about relationships and how to nurture them into something we both so desperately need. I have also learned how to reconcile and get things back to a much more fulfilling relationship than we could ever imagine. If you feel like reconciling at some point, I will become the best husband you could ever imagine. It is not that I will change who I am, but I will change how I give my love to you.

I do not want to dwell on what has happened, but concentrate on what could be. I know you have said that you do not want to go through this again nor neither do I. But, we won’t go through it again if we address the problems in the relationship head on with honesty and compassion.

Enclosed in this package is one of the many books I have read; His Needs/Her Needs. I think this book is wonderful and it describes us perfectly.

My love always,
Keith

Sounds more like a "I will never see you again" letter.
Use the example in SAA.

I am writing this letter to you and it will be the last one.
You are never going to write her again, even if you to reconcile?
Leave this out.

I have sent many emails and talked to you about our situation but it just not helped. Despite the hurtful things we have said and did over the last 6 months,
Leave this out.

I want you to remember me as the man you fell in love with
But that’s not who you are now. How do you want her to see you, as you were or as you can be/are?

At this point, it is best that we do not contact each other
Why?

I did not sleep for 2 nights after I seen you Sunday night which is why I called so angry Tuesday morning. I will never forgive myself for the horrible things I said to you that day.
Did you apologize to her for this?

I have also learned how to reconcile
You have learned that reconciliation is possible but not how to do it.

If you feel like reconciling at some point, I will become the best husband you could ever imagine. It is not that I will change who I am, but I will change how I give my love to you.
This is pretty confusing. I think I understand it but you need to make it crystal clear what you mean.
Also, when she ends the affair, you will resume contact. You don’t say this at all.

Enclosed in this package is one of the many books I have read; His Needs/Her Needs. I think this book is wonderful and it describes us perfectly.
I highly suggest you don’t send it to her, especially with a Plan B letter. Don’t try to educate her.

Thanks Chris.