I dropped Physics O level after the mocks - I had really struggled with it all through the O level course, and when I tried to revise over the Christmas holidays, the combination of not understanding the work, and the stress and worry of the upcoming mocks left me in tears.
I talked it over with my parents, and we felt that I was doing a decent array of O levels without physics, and that it would be better for me to drop one subject that was causing me real stress and anxiety and to concentrate on the other subjects, than to keep on struggling with physics, and risk it dragging all my other subjects down - so we decided I would talk to the school about dropping it.
The school were not particularly happy with this plan - I was in top sets for everything, and I think they wanted every good result they could get - if I dropped physics, that was one less result on the school's record, if that makes sense. I was told I should sit the mock and see how I did - so I failed the mock on purpose. I did a whole question we'd been told not to do, and tippexed it out, and made no effort at the rest. I got 29% - my lowest ever mark - and was told I could give up the O level, but could do the CSE if I wanted to - and I refused.
I have never been more relieved by a decision in all my life - I am sure it was right for me, and I got a slew of decent O level results, got decent A levels, got into nursing, and eventually went on and did a degree - losing that one subject has never held me back.
Only you and your dd can know what is right for her, but if this subject is causing her real stress, I would say let her drop it. She could always do it as an extra subject later on, if she wanted to - and all the Latin she has learned will still be of value to her. I did Latin for fun, at VIth form college, and I love the way that I can use that knowledge to see latin roots to words, and to help me work out meanings of words I don't know, for example. No learning is ever wasted, even if it doesn't have a certificate attached.