Been a working single mom, working married mom, and most recently a stay at home mom/engaged.
All have been hard in one way or another - I'd have to say HAVING KIDS IS HARD... whether you are at work or not, moms/dads still get tired, need time out, have selfish moments, their own life dreams - and kids bust through all of that whether you work or not... when I worked I paid someone else to cover the hours I couldn't. when I had a man at home plus work & kids, that meant virtually no time out from all - that was the most stressful.
I've recently accepted a new job. I'm engaged. Child is 12. The job is only 4x week so that I have 1 day off without kids or man. His days off are days I work and kids at school. We have a mix now where each gets their time out from the other... very nice!!! (not easy to do with younger ones, but at 12, its different than when they are little)