Squirrel, some time ago the man I thought was the one left me quite suddenly. And with that I lost the family I always dreamt to have. I could "understand" why I was better off without him, but there was nothing that could minimise the physical pain I felt at loosing him and his sons.
I howled, cried my head off, lost a couple of stones, but the worse was that pain, physical pain, in the chest that made me feel as if my heart had been taken out, chopped into little pieces and put back in its place, and it hurt with every beat. And that feeling was always there, at any time of the day. No metaphor here.
I tried everything to make that pain go away. It simply didn't. So at some point I accepted it was there to stay for a long time and that I just needed to cope with it the best that I could while it was there. This may sound strange but what helped me to keep my sanity was not to play movies in my mind of what was, could have been and weren't, it was already bad enough even without thinking of it.
Just remember, what you resist, persist. So allow yourself to grieve, but do NOT pitty yourself. You have done the best you could, this doesn't have anything to do with you (if so, your efforts would have worked). At the moment is impossible to feel this end is for the best, but in time you will understand why this happened.