What I find hard about being a parent is that you can never turn your brain "off".
Mine are 4 & 2 years old. I found the newborn years with them (as individuals) easy. Loved my maternity leaves, they ate and slept well and when I went back to work at 4 months, it was a good transition etc.
What I've found harder as they've both gotten older, is maintaining the headspace for everything that needs doing. And that's not necessarily the parenting part of it, but all the life "admin" that needs doing.
Remembering to license the car (which I forgot until yesterday and it expired 3 weeks ago), making notes of birthday parties, getting gifts, cooking, deciding what we're having for dinner, grocery shopping, grocery lists, walking the dogs, laundry, making lunch kits, remembering appointments, trying to maintain a decent social life, finding time to exercise, juggling a full time job, hearing "mummy can I have...?" constantly and the relentless chatter of a 4 year old with verbal diarrhea and a 2 year old who's favorite word is "no!" .
The parenting part would be easy if I could get rid of all the life admin in my head. The life admin would be easy if I didn't have kids. The combination of the two parts are what makes it hard. We're also in the process of selling our house and moving to a temporary home until our new house is built. Once we're settled house wise, I will be outsourcing as much life admin as I possibly can!