Of course the parent:child ratio is also important here. If I'd just had my firstborn, life would be much easier. I only have two, but the second has significant needs and requires high intensity parenting. It has been very hard to give both children the attention they need, while working FT so as to feed them.
I had this discussion recently with a married friend who also has dc, a wealthy husband and a PT job. We were talking about structural inequalities in getting to university and I said, "Of course the biggest privilege a child can have is two parents in a functioning partnership". She asked incredulously if I really thought that made a difference.
I had to spell out for her that lone parenting usually means less time and less money; it means not helping your child with their homework because you're scrabbling around cooking dinner, running baths, sorting out clean uniform for the next day; it means not taking your child for educational trips at the weekend because you're doing all the bloody housework and life admin.
And yes, I know that there are many married women who have it just as hard because their dp is an absolute plonker, but those mothers who get some respite probably struggle to understand what it is like to truly do 24/7. I'm 20 years in, and firstborn has gone to uni, so life is definitely easing for me. But the last two decades has taken a huge toll on me, physically and mentally.