Yeah, DD just turned 4 yo today, got a near 4 mo DS and it is EXHAUSTING! I'm more tired than my DD at 7pm than she is! I've heard it does get easier, but I'm dreading having to go through the 'terrible two's' AGAIN with DS having just getting through the worst of it now with DD, especially since DS is starting to turn into a 'Velcro baby', but does sleep through the night at lest.....
As much as my DH and I wanted 2 children, I have often wondered whether I can cope with 2 children under 5 on my own, (DH works 12 hour shifts in care, as will I when I go back to work after maternity leave).
I love my children, I would never go as far as saying I 'regret' having them, or wonder whether having them is a mistake, but I have come to realise that I underestimated just how mentally exhausting as well physically it can be, I don't think I was prepared for the 'constantness' of it. There's no instruction manual and NOTHING prepares you, no matter how many books you read, how many workshops you attend, or classes you take part in.
Others without children have absolutely NO IDEA how hard it is and how much of a struggle it can be, it is meant to be worth it in the end though.
Just wish we had more help. (My family lives 50 plus miles away, my in laws are willing, but have their limitations, FIL is 70, overweight, using a stick and uncomfortable around new babies, SIL has the 'ick' about poo and will probably help out more when DC are both older, but that's not when I need the help, it's now with the fiddley things, particularly with having mild cerebral palsy).
Think it's a just case of trial and error, perseverance, patience and hope for the best, but I definitely do empathise with what you're saying OP, no judgements here.