I think as you've said yourself 'exhaustion' is the key.
So let me join your vent. When you think of the hours we work, the pay, the recognition and status, the repetitiveness, the isolation, the life or death responsibility, the milk, tears, shit and vomit we mop up everyday, the fact, however physically and emotionally wrung out we may be, we can never step away for fear of hurting our child who we love most, these emotions make utter sense. Don't they? How crazy we are left to break in this way, and then be diagnosed as depressed or suffering from PND. As if our brains have come unwired all by themselves. Or that month upon month of chronic sleep deprivation can have any other outcome. Or that -our- need just 'to be' should become an unaffordable luxury once we have children.
I used to dream of spending a week in a hotel suite, where I would crawl under a lovely fluffy duvet while the birds sang outside, and sun crept in through the shutters. Twice a day, my children would be bought to me for cuddles and stories, before going back to some sort of competently managed and loving home life (managed by who I do not know!).
It is quite possible that no-one understands you. It is also quite possible that everyone else thinks you are having a joyous time and excelling at this motherhood game. It is possible that your husband dismisses crying or any other obvious signs of emotional distress as the natural order of mothers (rather than the natural consequence of the unhealthy demands upon them). Have you tried being this direct and honest with your husband, your mother, your friends?