It's always sad when you lose a colony ... it can happen so quickly that sometimes you just don't get the chance to get in quick enough to do anything about it. But .... nature has a way of weeding out those creatures that are not destined to survive in the long term and even our best efforts to keep colonies alive and thriving will fail.
If there were more survivors in the colony I would be agreeing with some of the other posters who have suggested ways of keeping a colony going .. indeed, if you only had two hives I might have considered some other options but you have good strong colonies outside of this one and they need to be what you should concentrate on now as winter approaches.
It's so easy to make increase in the spring - from colonies that you know are strong - that's what you should be looking ahead to and planning your numbers (and your kit !) for next year.
Nothing to beat yourself up about - show me a beekeeper who hasn't had a colony failure at some point and you're looking at someone who has never kept bees or is a fantasist ! Onwards and upwards ...