Originally Posted by
the1975jen 
Not to oversimplify, but I think the grief is just for what we thought we could be and have with that person, not necessarily sadness for what our life is like today .
This really makes sense. I think it's possible to mourn the loss of something that never existed, like the idea of a happy, healthy future with the person you married.
I remember feeling that way when my ex had a big stylish wedding with wife #2. It seemed so unfair that he got to be a big success in his new life post-being-married-to-me, while I was still sitting at home every night.
Now-ex-wife #2 and I have become friends and she told me that at their wedding both ex and his sister got completely hammered at the reception and she was terrified that her family would notice, which set the tone for their (very few) years of wedded bliss. That was her experience of their Instagram-worthy happy-smiles event. The reality is very different from the show.