I'm a young mum.
So many of the phrases you're using are making me cringe inwardly. You think you're ready for the changes a baby will bring to you. You think you're mature enough to sail through. You think you're as mature as you're ever going to get.
When that baby arrives and you walk out of that hospital with the car seat, life is going to hit you like a lorry. When you're sitting there, awake at 2 in the morning, you've had maybe twenty minutes sleep in total and you've got several more night wakings to go, you'll realise you're not ready.
When you're screaming and sobbing at your boyfriend for no reason other than you're so tired it hurts to think, still bleeding, leaking milk everywhere, still in a lot of pain, but it's your turn to feed or change the baby, you'll realise your relationship isn't as ready for a baby as you think.
When you realise that people are staring at you, and judging you, and when you tell them how old you are and how old your baby is, they're making an instant judgement on who you are and what you're like as a person, and you're hearing 'That's a shame, didn't you want to have a bit of fun first?' for the twelfth time that week, you'll realise that nothing can prepare you for it, and no matter how mature you think you are, it's really hard to deal with.
The last one is probably exclusive to younger and older mothers, but the first and second aren't. I know parents in their 20s and 30s who've experienced this, and I've experienced it myself, but I'd imagine, from what I've seen, it's a hell of a lot easier to deal with when you're a bit older.
I don't say this to be a bitch, I don't say it to make you feel bad - I'm giving you my reality, and the reality of most of the parents I've met and spoken to. From the way you've spoken, you DEFINITELY aren't ready.