Poor you, Penguin.
Eggy faced and mental
I'm not there yet in 11+ terms, some years to go, but I know there are days when I've listened too much to the school gate/ OH etc and then 'felt bad' when DC has not done too well in some homework/ workbook whatever. And then the inevitable guilt when I remember that he is just a kid and no kid gets 100% right all the time. Which adult does?:( Only to find 3 weeks' later, he's got some other really technical piece of, say, maths, that I was struggling with!
One bad day between you need not set the whole tone for the preparation, however. There must be some stuff she did in the paper which she did really well and you can praise her for that; and some stuff she needs to focus on - but that's what preparation is about: concentrating on the stuff that is trickier/ does not come naturally.
You obviously are a lovely mum, who cares for her DD, or else you would not be helping her to sit the 11+ or be on here expressing your feelings. Try to see the positives and remember that we are always on here for when it's a dip in that rollercoaster.