I’m going to say it’s probably something you can’t understand fully until you have children OP.
Household tasks & personal admin is one thing, but I never really understood the “mental load” before I became a parent because it was all just either my own things or my own houses things, it wasn’t a huge task and none of it particularly took up any headspace for me.
But once you have children it’s a whole other level and it does make it all the more daunting. Suddenly there’s another person and someone needs to keep track of their milestones, make sure they’re hitting them & if not figuring out how you can help them to get there, keep track of what they’re eating, making sure they always have something they can eat, keeping track of their clothes & shoe size then making sure they always have things to wear in the correct size, a whole other person’s doctors and dentist appointments, nursery/school admin, figuring out who’s going to do drop off, pick up, evening meal, bedtime, breakfast, they’re unwell, who is staying off work? Is it me? How am I going to arrange that with my employer, when will I catch that work up?
It really does become a mental load to carry and unlike when it’s just you and a partner you can’t just drop the rope because that most impacts the tiny human you are trying so hard to take care of.