Well all I can say is you're not alone and I know it helps me sometimes to know that others are finding life exhausting and impossible too. I wake at 4.30am most days and like another mum here has said, this is my only 'me-time', as I'm too tired at night to have any kind of evening and go to bed straight after my twins are asleep around 9pm.
If you think about it, humans are supposed to be raised and live in small tribes, where everything is shared, from the childcare to the domestic tasks and 'hunting'/ earning the money. So it's no wonder we single mums feel like it's impossible a lot of the time - it is! On the other hand, I suppose we do just cope and get on with it.
I've just told my children that I'm determinded to do one thing for myself this w/e - not sure what yet - but I've done nothing at all for me for months now. My whole life revolves around them and their needs, my self-employed business (which I run single-handedly, even down to doing the accounts) and basic urgent domestic tasks.
This leaves no time at all for me. I guess you have to get more of a ruthless mindset - like a real, assertive focus to TAKE some time for yourself - otherwise it just never happnes and there's always something else that takes priority.
Right this minute, I'm checking Mumsnet when I really ought to clean out the cat litter tray, get the twins to get ready fro bed as they're really tired but won't do it on their own - and begin their bedtime story etc etc.
But instead, I've snatched a few moments.
What I'd really love to have is a long hot bath and shave my legs - as I've survived on showers for the last 4 months, never getting time for a btah and you shoudl just see the state of my legs now!! On the other hand, there's no guy around to 'impress', so I suppose it isn't really my top priority!
Maybe you could just get more ruthless in being self-protective and demanding time for you? I need to tell myself this too! Good luck