Korra is tortured and poisoned, put through hell by the Red Lotus. She fought the mercury and even the pain she feels so much when forced into the Avatar State. She was nearly dying, However, the poison had taken a great toll on her, confining her to a wheelchair in a physically and emotionally weakened state.
In the episode, Korra Alone, Korra is heading toward the Southern Water Tribe, together with her parents, Naga and Kya, she would focus all her energy on healing.
Three weeks later, while staying at the Southern Water Tribe Royal Palace, Korra suffers PTSD and having a flashback/nightmare of Zaheer suffocating her, then wakes up, severely distressed and breathing heavily. Korra was being comforted by her mother, Senna, when she begged her to visit Katara. Senna doing her best being a supportive mother of a daughter who has been through more than anyone deserves to suffer through.
Flashbacks, nightmares, and severe anxiety are all common symptoms of PTSD, and this holds true for Korra throughout the season, as we see her plagued by flashbacks of her near-deadly encounter with Zaheer. Even though, which the various traumas from earlier seasons have done just that, imprinting themselves on her mind, brain, and body. She has experienced something incredibly traumatic, and she cannot seem to get over it.
At one night, Korra sits in bed and reads several letters written by her friends, she felt so dejected, depressed, tired and defeated. She cannot access the Avatar state and told for the past three seasons and few years that she isn’t needed in the world anymore…
Korra writes a letter to Asami and confiding in her about her fears that she may never make a complete recovery despite her efforts.
However, she realized that she is not ready to return, somewhere in a deserted area where she obtains Earth Kingdom clothes, sheds all her Water Tribe identifiers, and cuts her hair.
During her travels, Korra writes to her parents in order to keep them from worrying about her when she needed some time to challenge her own inner pain. She falsely tells them how pleased she is to have returned to Air Temple Island and reunited with her friends. Korra vanished without a trace when everyone thought she had been safe all along. When her friends and family found out that Korra had been lying to them about her whereabouts, they were deeply worried.
Ghost Korra is terrifying and is a representation of her PTSD, a hallucination despite feeling the pain she's inflicting. You can't escape her… If you try to hard, it will haunt you. Korra tried to beat her ghost self, but she loses.
It’s very emotional when Korra is suffering post traumatic stress disorder. She definitely went through hell and back and going through so very much of her hardship. Her strength isn’t just about how much she can handle before she breaks. It’s also about how much she must handle after she’s broken. She never know how to face her fears and inner demons.