So we know that Korra gets her bending back, but that doesn't make the tragedy of her losing them any less terrible. This whole episode is just one real punch to the gut. Amon (remember, Noatak? Hope you can keep up here) uses bloodbending during a battle with Korra and Mako and ends up severing her connection to her bending. You know, all those years she spent in, essentially, isolation? They're all worthless, now. Except--now, apparently, she's finally an airbender. Thinking she's lost it all is what moves Korra to connect with the portion of her powers that she hasn't been able to before. She worked through her spiritual block, but it took something awful to do it. Because before she realizes that she can airbend, Amon is about to take Mako's bending, too.
After it's all over, Korra goes back to find Katara, who tries to fix our gal. She can't do it. Lin Beifong, who also lost her bending is desperate. Pleading. Everyone is desperate. If she felt like a failure earlier, image how she feels now. She's just an airbender. She's not the Avatar. She leaves. And, look, you don't have to imagine how she feels. Korra rides away to a cliff, sobbing. She's been working toward completing her Avatar training for her whole life. Now, much of it is gone. Worthless.
But wait! This emotional rollercoaster isn't done yet. Aang shows up!
Aang, my boy! In losing everything, Korra was able to connect with her Avatar past lives on an emotional level. She called Aang there. And if anyone can help her, it's Aang. He gives her her bending ability back. And she goes into the Avatar State! Dear lord, everything was terrible but now everything is great.
That's not all. She's able to restore Lin's bending now, too. A miracle, I tell you. A miracle. It's the opposite of, say, mercury poisoning.
Captured by Zaheer, who is yet another evil dude, Korra is poisoned using mercury. The idea is that the mercury will force Korra into the Avatar State, and when she's there, they'll kill her -- ending the Avatar cycle for good. She initially resists the poison, but she see visions of the enemies she's beaten before. Zaheer first turns into Amon (not again), who is adamant that the Avatar doesn't need to exist anymore. Poison continues to spread, with more enemies urging Korra to surrender to the poison. To give in. And eventually, she does.
Zaheer and the other bad dudes strike, but Korra is able to reflect many of the attacks. And all this is good and sad, but the real horror comes in after Team Avatar defeats Zaheer. The poison really destroys Korra, despite most of it being drawn out of Korra's body.
We see Korra again a few weeks later, but she's very injured and depressed. Everyone is trying to remind Korra that she'll heal, but he disregards it all -- and sadly, it makes sense that she would. Being the Avatar is everything to Korra, something we learned early on. Much of Korra's sorrow feels similar to when she lost her bending; though now, she's mastered the Avatar State and still feels like a bad Avatar.
As it always does, it gets worse. Years pass, and the show moves into its next season. She's alone, removed from her family and friends. This calls back to Korra's childhood -- alone, dedicating everything to becoming the Avatar. She's in isolation, but this time, she's done it to herself. She's haunted by what's happened to her, and it's a huge hinder on her physical and emotional recovery.
Evil Avatar -- an apparition of Korra herself -- keeps bothering Korra. It follows her around the world, reminding her of the trauma she's been through. For me, this was one of the first times I'd seen a cartoon television show explore this kind of hurt and pain. To see someone as strong and powerful as the Avatar struggle really meant something to me, and as I suspect, a lot of others.