First off I am so terribly sorry and saddened that your W had to endure that awful experience. Many (most...all) of us have been hurt by another human being....but, I cannot even imagine seeing the 2 people you love the most in this world die before your eyes.
Secondly, I will start by telling you it is NEVER too late.....to love somebody (your W) unconditionally. Many of us (me included) have said we love our spouses unconditionally. The truth of the matter is that that statement was ALMOST true. We love them minimally-conditionally! I believe the most common condition is (was) "I will love you....IF you will love me the way I want to be loved". Can you continue to show your W UN-conditional love.....for as long as it takes....maybe for the rest of your lives? If you CANNOT....I believe it will be apparent to your W that you are only loving her to win her back. If you CAN, I think she will "know" that you can be trusted and you will see her return even quickly. What a paradox! You can only get her back if you love her without expectations!! You will lose her if you try to win her back.....
If I were you, I would pray about whether to write that letter. I had a similar moment which actually came long before my W started showing any signs she was willing to reconcile. She had told me that she would NEVER trust me again.....and that she wouldn't even DATE me if she met me as a stranger because she felt no attraction to me. She had confided that she believed her "healing" would take years! I didn't write her a letter...I wanted her to see my heart! I started by telling her that I used to say things like I was about to say so she would return something to me....even if only an "I Love You" (in other words, I was being needy). This time (I said) I don't want you to say or do anything. I told her that HER healing was the most important thing to me....MORE important than the restoration of our marriage. I told her that I love her from the bottom of my heart....and I then I said goodbye and left. I accomplished EXACTLY what I set out to do....let my W know that I would do anything to support her healing....even give up on our marriage restoration if that would help her. What was the reaction? Well, not much visibly....but, 4 months later we were reconciled. How long could I have kept loving her unconditionally? I will never know....although I felt like with God's help I could have kept on FOREVER!
I am praying that God gives you that same strength and depth of love for your W.....
Praising God Daily, Remaining "FaithfulH" Me: 62 W: 62 D:33 S:30 & 31 Married: 40 Years BD: Sep 2006 Piecing: May 2007 2nd BD: May 2014 Working On It: Today