I went from having no kids to two (twins) and I can’t even put into words what’s different because it’s literally everything.
We are in a more extreme position than most as we have no family around and if I could give one piece of advice - if you have family and can move closer to them, do it! Things you take for granted (being able to just pop out if the need / mood takes you, going to the cinema, going out for a meal, going away for a few days to visit someone, going on holiday, working, bingeing on a box set til 3am) suddenly become impossible. Getting out of the house is a military operation.
But I am completely different too - happier, more content, my house is actually clean because I have to clean it otherwise they’ll eat whatever they can find on the floor and injure themselves with what they leave lying around.
It’s easy to visualise all the negatives, all the things you can’t do any more, but you can’t visualise the positives - how they’ll make you feel, watching them turn from tiny babies into little people, the joy when they come over to you for a cuddle because they love you, when they do something new for the first time, when they find something completely banal hilarious. They are absolutely awesome. I am bloody exhausted but it’s worth it.