Lots of good answers so far!
I don't think anything can really prepare you for the emotional burden of parenting. As others have said, there's a relentlessness to the level of need that babies and children have for their mums. Except for when you are asleep (and you'll be doing A LOT less of that than you used to), literally 24/7 you are thinking about or attending to their needs, their routines, their health, their development, or being 'on call' in case they need you. You never really switch off and completely relax. You're likely to be the 'default parent'.
And - this may be controversial - very often there's a frustration of feeling that even your partner does not totally "get it" when it comes to the emotional burden. After the first bit of paternity leave, Dad usually goes back to work and essentially does not have to think about baby for eight hours a day, five days a week. In my experience, even when mums go back to work, they are still the 'default parent' who takes main responsibility for ensuring childcare is meetings baby's needs, who stays on top of vaccinations, and health checkups and preschool enrolments and playdates and activities and what new clothes and books need to be bought, and organising birthday parties, and reading up on how to deal with developmental things like tantrums and discipline and child safety, and so on and so forth. It's exhausting.
As others have said, so much makes up for all this, but it can be a real shock to learn how easily the strains of parenting can test you emotionally. I am generally a pretty calm and collected person, the sort of person people come to for help to sort stuff out when the sh*t hits the fan, so I was stunned at how short my fuse became when I was sleep-deprived and independence-deprived, with low blood-sugar because the last thing I'd had time to eat was a piece of toast with peanut butter on it 10 hours earlier, and frustrated because I'd been trying to get a baby to go to sleep for three hours and had completely run out of ideas on how to get that to happen.
There have been times when I truly understood how people get to the point where they shake babies. Before motherhood, I thought those people were monsters. Now I just feel grateful that I have the benefits of age, and temperament, and family support, and no financial stress, and no mental health issues, and no anger management issues, and all the other little things that can add up to make the difference between someone who cracks from the stress and someone who doesn't.
For me, the more time I spend as a mother, the less judgmental I become towards other mothers, because every day you learn more about the challenges parenting brings and become more open to the idea that people are just getting through a tough job, trying to make the best decisions they can.
But of course, it's the hardest things that are the most worth doing. You're proud of yourself when you climb a mountain, not when you climb your front steps.