You aren't suddenly thrust into having to care for and manage a howling toddler. A newborn is for the most part, not that difficult, once you account for the lack of sleep and your tolerance for crying (and how much your baby may cry for that matter- mine didn't much). Of course there are things that seem BIG and you are all encompassed by it while you're doing it. Slowly but surely though, your newborn infant will start to be more interactive. They roll over. They sit up. They look at things. And you, as their parent are right there with them. In fact, the reality is, you will likely miss many of these changes as they blur into a happy memory of newborn haze. The reality is, in the first six months of your child's life, you are becoming an adept parent just as your child is moving from a newborn to a baby.
The same transition happens from baby to toddler. Children don't one day wake up and start running. There is a learning curve. And again, you are right there by their side, watching their actions, learning what they do so you can be there to support them. The change in some ways is fast, but in other ways is slow and incremental. So slow, that you don't really notice, but again, your skills at managing your child are keeping up with their development.
Does that mean there aren't moments of desperation or exasperation? Of course not. But they aren't the end of the world. They're a moment. Moments pass.
It's funny though, because as much as I understand this paradigm, thinking about the reality of having two children based on having one, is one of the key reasons I don't really want another child. Because I see what it takes, and I don't think I have enough to go around for two. At least, not to do it the way I want to do it. However, the reality is, if I had another child, I would adapt day by day until my new reality was commonplace. I still believe there would be more moments of exhaustion and exasperation, and I still know that financially it's not really what I can afford (or consequently what I would want if I did). But it's the same principle. Everything about children seems overwhelming until you have one. Then you just take it it day by day. And each day isn't actually all that hard upon reflection. That's how you handle it.