Closure... But you're not going to get it from your Ex. So what to do?
Gift yourself closure. You did what you could. But it wasn't enough. The reality is nothing will ever be enough for her. No matter what you did in the past or promised in the future would have changed the outcome.
Ponder the Five Stages of Grieving a Loss. There is denial, anger, bargaining, depression and eventually
acceptance. It was what it was, it is what it is. You put forth valiant effort, left no stone unturned and yet the marriage still failed. Not your fault. Recall the task of Sisyphus was to push a boulder up to the top of a mountain, watch it roll down and then do it all over again every day.
I recall a few who mentioned writing one last letter. But like you they never sent it. Some memorialized the sad end by lighting their letter, almost in a ceremony, to Let Go and Move On.
Ponder what you will do in the months and years to come, what you will accomplish, the joys to be treasured...