I've come into this one a bit late after reading most of the pages on here,but I thought I'd add my experiences.
I certainly use the term 'mental load' because it IS a load.I'm 'Thinking for Two' as my husband has a progressive neurological condition which,in its current phase amongst other things,is making him rather forgetful and unreliable.
I have to reduce his 'mental load' in order to keep him on an even keel.This means initiating all work/maintenance done on the house and all domestic arrangements.There's a plethora of hospital/general medical appointments to juggle as,like buses, they seem to come all together,and they're all in different clinics,surgeries and hospitals.
Despite the fact that everything is written down in big letters on a big chart,he still forgets things.
I can't trust him to remember a verbal instruction.
I had to be very organised in my professional life,and indeed sought counselling in the later stages of my career as this mental load together with managing DH's condition sometimes made me feel as though my head would explode!
The work element has now been removed as I'm retired so that's helped enormously,but now that there's more time available to devote to much-needed work on the house (eg an extension to provide downstairs facilities for DH) my head is still crowded.This is work that will be carried out by others,but I still have to initiate and plan it all.
As lots of PPs have said,it's not the doing,it's the thinking.
The 'Doing' can be paced.I'm one of those retirees mocked in past MN threads for only doing one 'Big Thing' a day.So be it.It means that we both stay calm and things get done- eventually.
The 'Thinking' on the other hand takes up a great deal of mental and physical energy as my mind will often be buzzing at night and sometimes I just cannot sleep thinking about things that I know very well can wait until morning.It fills my head constantly.Strategies such as burying myself in a good book do help,but my mind will wander,especially if I read something that triggers a thought about something I need to do!