Sorry to butt-in from Essex.
Don't be downhearted - after all your DD isn't and that's so important at this stage.
I always think it's an amazing show of strength when children are self-motivated in such a way. So many DC's have to be encouraged to do 11+ prep, with varying levels of unwillingness and lack of self-motivation. If your DD still feels confident, then in your shoes, I'd encourage her every step of the way and be there if (and let's hope this won't be the case) the outcome isn't what you hope for.
They all have a meltdown at some point and if you can, have a discussion with her about where she feels her strengths and weaknesses lie, in the light of doing mocks. Have a chat with her tutor and see if you can draw up a plan of action.