We don't know what misery she's been going through.
Of course Catherine is lucky to have the best possible care and the luxury of time to recuperate (no-one would argue with that), but there's another difference between her case and that of some other cancer sufferers: the minutiae of her appearance will be scrutinised mercilessly and intrusively (and tastelessly, by some) in the press and on social media. Most of the rest of us at least don't have to deal with that. I say this simply to suggest why she may not be working as many engagements as some people might wish. The expected standards of personal presentation and behaviour, in terms of keeping smiling, chatting to all kinds of people and the travel involved might not be compatible with the rigours of cancer treatment.
I feel for everyone who's going through something similar - but I'm sure everyone's case is different and it's impossible to generalise about how she might be feeling or how long her recovery will take. I think a pp might have a point - very sadly - that the cancer might always have been more serious than has been reported. Keeping it quiet and vague and playing it down is the only possible way (in today's world) of protecting the children from hearing traumatic news while they're so young. I really hope it's not as bad as that, and that she's on the way to recovery.