The world of Avatar: The Last Airbender displays some of the most impressive elemental abilities in fiction, with Avatar as the master of them all. One of the most powerful tools at the Avatar's disposal is the Avatar State. The Avatar State allows the Avatar to access all of the combined experiences and powers of their past lives. But without a complete mastery of the ability, the Avatar State can be incredibly dangerous to both the Avatar and their companions.
In order to not put his friends in danger and have a better chance at defeating the Fire Nation, Aang began his training to master the Avatar State. His teacher, Guru Pathik, told Aang that he needed to unblock all seven of his chakras in order to control the Avatar State. Aang managed to unblock six of his chakras but was unable to unblock the seventh since it required him to let go of his earthly attachment to Katara.
Pathik warned Aang that if he failed to unlock the final chakra then he would never be able to enter the Avatar State again. During a fight with Azula, Aang decided to enter the Avatar State but was struck down by Azula's lightning just as he gained control. The resulting injury prevented Aang from entering the Avatar State for the rest of the series until a protruding rock jabbed his scar and allowed him complete control of the Avatar State.
This sequence of events left many fans confused. How does a simple rock reconnect Aang to the Avatar State and how was he able to control it when he never unblocked all of his chakras? Clearly, Aang did not let go of his earthly attachments if he still loved Katara, right? This is where Aang and many fans misunderstood Guru Pathik's teachings. Pathik was not teaching Aang how to remove emotions from himself but how to not be controlled by them. Fear, guilt, shame, grief, lies, illusions, and earthly attachments all block the seven chakras, but Avatars before Aang have mastered the Avatar State while still experiencing these emotions. Letting go of one's fear or guilt does not mean that they never feel those emotions again. It just means they won't let those emotions influence their actions.
When Aang was fighting Azula, he took the time to meditate and open his seventh chakra. He let go of his earthly attachment to Katara. This doesn't mean that he no longer loved her or cared about her. Other fully realized avatars like Roku, Kuruk, and later Korra were able to control the Avatar State while being in love. Aang's decision to let go of his attachment to Katara just meant that he would not allow his feelings for her to hinder his job as the Avatar. Once Aang opened the seventh chakra he had full mastery of the Avatar State until he was mortally wounded by Azula. Azula's attack had resealed all Aang's chakras. Although Aang had mentally succeeded in his training, his physical wounds prevented him from accessing the Avatar State. It has been established in the series multiple times that physical wounds can block the flow of chi even if the person mentally has mastered the ability. Combustion Man was unable to control his bending after being hit in his third eye, where the Light Chakra is located.
Aang's blocked chakras would explain why his character seemed to regress in Book 3. Aang was terrified of confronting Ozai during the eclipse even though he released his fear of Ozai during his training with Pathik. The same situation occurs when Aang has trouble learning how to firebend because of his past experience burning Katara. Aang released his shame over that event during his chakra training, but it resurfaced because his body was physically blocked.
Aang's physically blocked body would continue to be a hindrance until his scar from the lightning strike was stabbed by the rock. The rock acted like a form of acupuncture that released all of his chakras. This returned Aang's body back to the moment before his injury and realigned his physical form and his mind. Aang had already mentally done the work of mastering the Avatar State so when his body was restored he had complete control. Avatar: The Last Airbender is a series that stresses the importance of both mental and physical health. The mental effects and the physical and vise versa. In order to be your best self and achieve your highest potential, both aspects need to be in balance.