That you'll never feel 100% healthy again, even at your best, pre kids you would think that you were running on 60% and spend the weekend on the sofa recovering.
That absolutely NO ONE cares about the above and you'll discover you have a new found capacity to power through and carry on that you never had before, and that it IS worth it. (although sometimes only in hindsight)
That everyone who was a parent before you really Meant the stuff they warned you about. I heard it but it never really sank in until after DD arrived. I remember thinking, why didn't anyone Tell me!? They did, I just shrugged it off.
You won't 'do it differently ' when you have kids. Your idyllic view of parenting that you had before you actually were a parent will quickly dissappear and you'll do what is best for you and the kids, sometimes that means plastic tat or screen time ( the old me is hoiking up her judgy pants saying ' yes but no more than twenty minutes a day '!)
Even if the parenting books are legitimately helpful, at 5am if Gina Ford/Super Nanny etc walked in you would cheerfully punch them in the face. Probably with a sicky muslin wrapped around your hand.
Your heart will multiply in size like one of those sped up films of a massive oak tree growing from a tiny acorn. There is so much Love for your baby in your heart. ( I always rolled my eyes when. I was told this pre baby)
You might become addicted to smelling your newborn and become irrationally angry with anyone who holds them wearing heavy perfume for interfering with that intoxicating scent.