I suspect, when we once loved someone, it's hard to let go of the dream we once had of being together forever. Even though you've parented alone for years - maybe it's hard to lose the dream that he once was "the one" - even when it's obviously the best thing to do. Weren't you very young when you got together? You had five children - you wanted it to work out and did your damnedest.
The death of a dream is still a death.