I’m sorry you’re in this awful situation. 
Would you be better not starting your own business in order that you can claim benefits? What kind of business is it?
Can you go to a food bank?
Could you take in a female lodger?
Could you be a dog walker?
Is your house bought outright, would you consider selling and downsizing?
Don’t give up.
And if you get to a food bank and eat more yourself it may give you more strength, and help you think clearly.
If you make a mistake with sleeping tablets and overdo them and end up ill in hospital your ex could be expected to take your child on, so you need to stay healthy for her if he is not a good guy, which I imagine he isn’t or he’d be paying for her. She’s better with you. You need to be ok fir her.
It’s not you who has let the child down it’s him.
This is a temporary situation. Things will improve. We’ve all been there.