Once you get pregnant you will be worried and scared for life and it doesn't end when the baby is born healthy. As they grow you will worry about something happening to them, that they are developing correctly, eating the amount they are supposed to, etc. I've come to realise I will never stop worrying no matter what age they are.
The mach nurses tend to follow books to a T so don't always worry if they say you're baby should be doing this and it's not. It doesn't mean there's something wrong.
It's okay to stop breastfeeding if you have to for whatever reason.
Things don't always go to plan.