I’m confused. Why would you be “sorry” that my D “couldn’t handle,” as you put it, a sixth AP course during the same semester she was taking 5 of them, had already finished another 5 the prior year, was applying to colleges, continuing her international level extracurriculars, and winning awards – all of which resulted in 3/3 of the Ivy League U’s she applied to?
It seems as if you think she did not have at least the same level of internal drive your own child/children did. That would be doubtful. 
It’s just that overload on the human brain and body is a reality for even the most capable student,-- regardless of the level of drive – and exceeding that is not something which is required to prove that one is capable of high level work. Perhaps I miscommunicated?
Whatever level of intellectual thirst any student has – my own, yours, anyone else’s here, no one is so super-human as to deny physical limits.