Dio Exodus
Korra will bounce back; I'm not worried. This is literally the third time life has broken her; It happens every season. Well the first time Season 1, it was there for about 5 minutes before being solved, not much of a down time (and seemed nulified by the poor way Season 2 had her act out as if season 1 never happened). Season 2 it was rough but in the end it only lasted for a short bit but again was resolved before the end. Sure more lasting with the lack of her connections but still not that big.
Season 3, she is STILL out of commission 2 weeks later. Shes refined to a wheel chair and likely partially blind (assuming by their choice of doing her appearance and assuming she had mercury poisoning). On top of it, Zaheer in a way is shown to be right as far as the world not needing the Avatar. Even with her out of commision, the world seems to be able to handle itself. The air nomads in a way are "replacing" her role in the world. Its really showing her how much little relevance she has anymore, something so adamant about "I'm the Avatar" and being prominent about herself its a harsh blow.
Its not a simple 'bounce back' and her very existence is in some questioning if its needed anymore as all the roles the Avatar was suppose to be there for seems to be practically gone. I'm sure she will recover from her sickness, but mentally she has to deal with the fact her roll in the world is not the same.