I sat there staring at your post for a good ten minutes and still find myself at a loss on what to say. This is a whole new layer of grief that I am not even sure a book addresses. My father died when I was 10, my mother and he had been divorced (married other partners) since I was 2. After I was coming home from the funeral I saw my mother weeping. The fact was, she loved that man once, she loved him enough to make children with him and that meant something.
So grieve however you need to grieve, I don't think there is a manual for this one so don't worry about following a rule book. I am so sorry, my friend.
M 10yrs T 13yrs BD #1Oct 2015-PA between WAH and COW BD #2 April 2016-WH resumed PA, she broke it off Jan 2 2017 WH says he wants divorce April '17-Letting go 2018 D busted DD8, DS6, DS3