I feel for you OP, it must be very hard. But I also agree with the advice here, to focus on all the positives and enjoy your dd's childhood.
When it comes to children, it's not quantity that's important. If your dd has a lovely upbringing and is loved and valued, you will have a happier family than one with 3 or 4 kids who don't invest that love and care.
It may also be worth pursuing a few dreams which would be less easy with more kids. Think of things you can afford to do with one, or which are more practical with one, which you couldn't do with more. Maybe some city trips abroad, where you won't end up dragging 4 whiny kids around and can really enjoy each others company. You could also capitalise on the fact that you don't have the problem of vastly different ages to cater for. And in a few years you'll be able to do mum and dd spa days!
We have two kids, and are very happy with that, but when I look around, if I envy anyone at all, its not the friends we know with 3, 4 or more. If I had any other scenario I would go for a single child, because all the ones I know are really calm happy families. Not saying the others are all unhappy, but I've seen first hand the sheer work load of balancing lots of different needs. If you were to fall pregnant again now, you would have a massive age gap anyway which would restrict your and your dds lives, so I would definitely try to accept how things are and enjoy it