Congratulations, Daisy.
I remember going for my first scan feeling sure I would be told there was nothing there even though I'd had the positive test and seemed to spend all my days either eating cheese and onion crisps or being sick. like you, I just couldn't quite believe it had actually happened and I'm sure a lot of women feel the same way.
Don't forget too that during your first pregnancy you would be able to take it a bit easier and get plenty of rest. This time you're running round after a two year old who won't let up on all the things he wants / needs doing , just because you're feeling sick. During your son's nap time- if he still has one - stuff the housework for a while and take the chance to put your feet up.
Best wishes for your pregnancy.