Yawn, not another one of these threads.
Why are there so many cool girls who are so annoyed or deliberately obtuse at the concept of mental load? It's not a difficult concept to grasp.
If one person in a relationship becomes the "project manager" of the relationship/family/children/home/all domestic stuff, the workload is lopsided and it creates resentment.
In a lot of modern households both parties are working (often full time). Quite often a woman will be working full time and doing the bulk of the childcare and housework. On top of this she is very often expected to do the planning and thinking (booking GP appointments/dental appointments, remembering homework and parents evenings, booking vet appointments, booking holidays, checking when various types of insurance need to be renewed, booking a cleaner, planning holidays, booking rail tickets, remembering to buy birthday cards, etc etc etc).
This is what mental load is. (And before anyone piles in, no it's not about paying bills). I don't understand why people are so determined to pretend they can't understand this.