Mind if I ask what your mite load looked like last July/August? It seems, from what I have read, that is the time that you have to get the mites low enough to build healthy winter bees. Where these hives first year? Where did they come from? Swarm, split, package...etc.
Just kinda wondering... Sorry for your losses and if there is a lesson in it I hope you learn it (and then pass it on to us new bees)....
Did not check where you are at but in Washington State you can send dead outs and whatnot to WSU for free testing to find out what diseases, if any, they were suffering at the time.
Spring will come and with it a chance to rebuild. I guess I am just an optimist that way. Good luck to ya.