I'm a Step Dad to two (7 & 11 now) and dad to a 2 year old. It's been bloody hard since our own came along (we got every other weekend off before that!) but one thing I've long thought has really become a solid belief:
Mums aren't meant to do this alone. Even good, functioning and equal contributing couple aren't meant to do this alone. They're meant to do it in a tribe. From an evolutionary perspective, the touched out mum should have an army of supportive tribe members ready to pass the kid around so nobody is going insane. The whole nuclear family thing is a construct that is all fine and lovely but deprives a mother of the support network she should have and shames her for needing it.
We're doing this on hard mode, against our design.
Personally, I went from zero kids to three (in all practical terms)in the space of a very few short years. I do my share. I do nights and have done since the second breastfeeding stopped at seven months. We sit down together, finally, when everything that needs to be done is done and neither slack off before that. And sometimes I 'regret' it. But because it all happened so quickly (relatively, I'm in my early forties and the years don't seem so long...) I still remember what was shit about no kids. What was missing - for me, totally understand that for some people kids are not something that will ever be 'missing' for them. That makes it easier for me to have context when I 'regret' it.
I suppose it helps that as a couple we never had the carefree childless years to miss? But I remember the first time I saw her with her kids, the way she looked so complete with them, laughing with them and so naturally enjoying their existence. It was later that I'd learn about her locking herself in the bathroom at times and just crying because at that moment she just hated it, so so much. Neither bit was false, the joy with them when I saw her or the utter misery in the bathroom. I just believe that in the latter moments, we evolved to have a mass of people around us to help.