I knew my parents were getting divorced, and I was just 3 at the time, just to give you an idea of the vibes kids can pick up on, as mysonsmummy says. You are not a bad mother, you're just struggling with a nightmare set of circumstances, as work stress has uncovered lurking home stress to the point where it can't be ignored any more. I would definitely get your mum to take dd out for the day / part of the day and get yourself a little 'me' time - not even just to think but to have some quiet time to chill out. You will feel stronger for it.
There's a Chinese proverb about if you don't know which path to choose, do nothing, and more will be revealed. (Am not Confucius, read this in the Michael J Fox auto-biog I think!) - it sounds like that may be your best option right now, if you can find a way of making the job bearable in the short term.
In terms of money, dh and I are unusual in that I work full-time and he is a SAHD. So by rights (based on some of the horror stories I read on MN, not just this one). I should be:
- demanding to keep a massive share of the income for my personal amusements
- doing nothing around the house
- buggering off every weekend playing golf or indulging in other 'hobby' that excludes my family.
Somehow ... it doesn't quite work like that when the roles are reversed. We don't actually have a division of spending money because dh is chronically ill, so has left the house once in the last four months without me, hence I do spend more on bits because I'm out more, and if we're out together, I pay.
But if I thought I was spending 450 quid of our money (I earn it, dh enables me to earn it) on myself every month, I would feel like the worst mother alive.