It's exhausting!
I found working so easier especially as my dc get older. I could get 5 minutes of mental space. I could concentrate on one thing at a time.
Firstly I find I'm 'on duty' all day every day, day and night. My attention is constantly being drawn from one thing to the next and I feel mentally alert all the time and physically it's actually quite demanding.
There is no difference between any of the days or weekends, it's relentless and never ending and weekends are actually harder because everyone is at home and requires feeding and tidying up after 
.
I rarely sit down. I'm on the go from the moment I wake up to the moment I collapse in bed and then during the night as well. People need rest, sahp get very little rest.
I find not getting any job finished in one go and everything taking hours longer is frustrating and stressful and tiring. Also the amount of work I have to do! Constantly rushing about. Multi-tasking, no lunch break or private quiet time.
I keep reminding myself that I'm actually doing two jobs at the same time. House keeper, childcare and all other things added to it.
It is hard. It's not very stimulating either which doesn't help. I feel much more energised after a day of doing something different or adult related.
I find very few people understand my view and a lot of people think working is harder. I have a stressful, active profession yet still find SAHP is exhausting.