There are some things in motherhood that you can prepare for. You can take a class to learn how to swaddle or to change a diaper. You can go online and research things like developmental milestones. You can read a book for topics like sleep training and potty training. There are many, many more things that you will encounter as a mother that nothing will ever prepare you for.
Even if you have friends and family that had children before you and have told you about some of the hardest aspects of motherhood, you don't quite understand them until you experience them on your own. Like that no matter where you go, you will carry your child with you. Even if you aren't physically with your child, you will probably always think about them. If you aren't thinking about their well-being, you may be thinking about things that you need to do for them or items that you need to purchase for your child.
You will find that what you envisioned motherhood to be like and what it actually is to be completely different. Many of the things that you said you would never do as a parent, you now do. That can be a very humbling experience when you realize that raising your hypothetical children is a lot harder than raising your actual children. You will feel like a failure quite a bit. You feel like a failure if something doesn't come naturally to you. You will feel like a failure if you're unable to accomplish everything in a day that you set out to do. Not because you are but because your idea of what you should be able to accomplish as a mom is probably completely unrealistic. Also too because you aim to give your child nothing but the best and anything short of that makes you feel like a failure.
You will neglect yourself. You will find that most of your meals are eaten cold, if at all. You will discover a new ability to go days without showering. You will literally find yourself transformed into an episode of what not to wear and won't care because, in the grand scheme of things, that's not what matters to you anymore. You will be less carefree. Nights out will not have the same level of fun that they used to because you won't be able to fully let go and enjoy yourself the way you did before having a child. Even if you are able to let loose, you may find yourself worried about how your daily responsibilities will resume the next day as they always.
You will worry incessantly about whether or not you're making the right decisions. Nothing brings about self-doubt more than motherhood. The self-doubt will range from simple things like whether or not your child is getting enough food to major things like how each and every one of your actions will impact your little one's future life.
You will have to constantly adapt. Less often as your children get older, but in the early days, it feels as if the minute you're getting the hang of it, your child changes in some way or another and you have to change with them. Motherhood is constantly adjusting to a growing and evolving baby. Motherhood is permanent. It is literally a 24-hour job with no breaks. There will be many days when you are sick but to your child and your family, that means nothing. There will be other days where you're just feeling moody. In your pre-kids days, you may have opted to stay inside the house all day and binge on your favorite TV show but after kids, that option no longer exists. Even if a serious life event like a death happens in your life, you may not have the ability to mourn it as deeply as you would have pre-kids because, for your family, life goes on.
Motherhood is a complex role because it can be extremely difficult, yet rewarding at the same time. You can experience complete bliss and wonder how you ever lived without your littles one minute and be completely miserable and miss your old carefree life the next. It is literally the only job that you'll ever have where you have no training and no experience and at the same time one of THE most important jobs that humans will ever have. And that is why it is one of the hardest jobs in the world.