Ha ha there was one word that sprang to mind before I flicked through the other responses... it cropped up a lot. Relentless.
Think about having someone you care about imensely being virtually pinned to you 24/7. Before kids you can say 'hang on, give me a mo I need a wee/a minutes peace/ to sit down....' That just doesn't happen any more. Every moment is amazing but you have to be there pretty much every moment. If you take a break (lucky enough to have DP or parents, PIL, friends etc to help out) you then you just swap tiredness for missing DC and a twinge of guilt for not being there.
In the early days in was BF. Being pinned to the sofa/ bed etc for hours on end. Then when moved to bottles it was the constant washing and sterilising and boiling and measuring. I also had a bit of paranoia (not uncommon it would seem - as I found out later) that DS would stop breathing. So I found sleeping really hard as was constantly worried he'd die while I was asleep. Irrational I know, but that's the weird kind of stuff that can go on in your head when you are completely responsible for your tiny little person, even if always previously been a very level headed, practical, rational type of person!
Then they start interacting and want your attention CONSTANTLY! I ended up using a sling from about 7 months as he would scream if he couldn't see what was going on and wanted to be held all the time. Going to the loo or having a shower becomes a mission.
It was at 6months to walking I think was most draining/ tiring with weaning, and still on milk and crawling and wanting to be into everything, wanting me to interact ALL THE TIME and me feeling the need to socialise with other mums to keep sane....
Then there's going back to work. My god it's hideous. Get up at cracking of dawn to try and get a grumpy baby and yourself ready to leave the house as some ungodly hour of the morning to have the emotional roller coaster of leaving DC at nursery (often screaming for you) to then make a mad dash for the train/ sit in traffic to try and make it into work by the skin of your teeth (with feeling of guilt for not being able to make it on time like you used to) to then try and prove yourself as still committed to your job despite thinking about DC constantly. Then start the mad dash home and nursery pick up, try and squeeze I some quality time with DC before everyone melts down with tiredness. Then bath for them. Start cooking dinner for you. Eat. Sleep. DC wakes up. Try and sleep. DC wakes up again - been sick, needs milk, wants to play, tell you about monsters. Try and sleep with an overactive child in your bed. Alarm goes off in what feels like middle of the night...
From when DS could walk a started to find it get a bit easier. He wasn't as demanding as could now be a bit more independent, which then gave me a bit more independence. Then he started talking.... "mummy, mummy, mummy, mummy, mummy, mummy, mummy, mummy, mummy, mummy, mummy, mummy......"
Yeah relentless.....