I've been been reading through this thread and much of it has resonated with me, I don't have anything to add in the way of advice but hope that you'll take comfort in the fact you are far from alone.

Bugs do creep into marriages as eskielover says and I think the most common one is not really truly listening to each other. It sounds so obvious, but after many years together this is something that can creep in without anyone realising until a crisis point occurs. It can sometimes feel like things said and done have come out of the blue but if you step back and disassemble the situation then you can usually see the pattern was building. It's horrible to reach a crisis point, I know from experience, but the darkest hour is before the dawn.

My best wishes to you and your family.