[Waves to Whoknows
]
Op - you sound really lovely and very sensitive to your dd's needs.
I absolutely agree that, as dc grow, we need to respect their autonomy and choices but with the caveat that dc tend not to be great at looking far ahead in to the future , and so, occasionally, we need to help them with that. It sounds as though your dd is very mature for her age though whereas my dd tends to be a bit impetuous at times and only thinks about the here and now, or at best, next week! 
This is particularly relevant to ballet I think, because, unlike musical instruments, it is more difficult to pick it up again once you have stopped.
We went through a similar situation when dd (nearly 13yrs) was nine. She had, up to that point, in the main, really enjoyed ballet (she isn't doing it vocationally or anything, just four or so hrs a week at a local school).
But then she went through a bit of a blip at school when she was struggling socially and with various academic issues and it all got a bit much. So we gave up all extra-curricular activities for a year (at that point she was also doing tennis and regular horse-riding) to ease off the pressure. I'm glad to say this strategy worked and having missed 8 months or so, she returned to ballet lessons the following September.
This has, overall, worked out well as she is now back at ballet, en pointe and enjoying it hugely!
However, however, it hasn't been all plain sailing because she is now one year behind her ballet classmates with whom she had made friends previously (she has one particularly close friend in the class). This hasn't been too bad this past year because both her level and the level above are taught together (with variations). But next year, when she is with smaller dc, she will really feel it I think.
Also, she has shot up massively in height in the past year, and developed overall, and this is obvious when they all dance together as she towers over the others! This means that during performances, she tends to be placed at the back!
The reason I am boring you with all of this (!) is that I think it would be quite difficult to give it up at a later age and return again - I think you could get away with it at 10 yrs and probably return in a year - but no later than that. And definitely not advisable once you have started en pointe.
I think it is totally normal to have doubts and major wobbles at our daughter's ages. I always remind my dd that dance is such great exercise and a brilliant social skill and its a great way of maintaining fitness throughout your life. Also, that if you can do ballet, you can turn your hand to almost any sort of dance.
Hope it works out for your dd whichever decision she takes!