Don't beat yourself up too hard. It's often hard to switch out of "mum" mode and into "nurse" mode - when you're trying to think like both, it's easy to miss obvious things, although given her age, and the lack of a family history, asthma probably wouldn't be the most obvious diagnosis.

One thing though - don't beat the children's nurses up too hard about trying to keep your little one asleep when they did the spacer. It is possible to give a spacer to a sleeping child without waking them (I've done it) - they were probably just trying really hard to get you both a bit of uninterrupted sleep.