I think with any relationship that was deeply important, no matter how long it lasted or whether, you were married, to hear that the someone who was once the object of your affections has died, can affect one deeply.
Long ago and far away I fell deeply in love with someone, but it was not to be. By chance a few years ago I heard he had died and I read an obituary published in his professional institutes journal, which showed that my memoriesof him had not embellished him and until the day he died, he was the person I remembered. I mourned him for some time and still think of him now and again.
Mourning him, even though I fell in love again and have been married to my DDH for over 50 years.
Of course you will mourn someone you were married to for eleven years and who fathered your son, even if the relationship did eventually fail.