Katara was barely aware that she just walked into her room and sat on the bed, her head spinning and her mind in a war-like state, all her thoughts screaming in her head.
How could everyone trust Zuko so quick? She asked herself over and over again.
They can't possibly have forgotten how he chased us around the world, making our life hell. How could they believe he suddenly changed his mind?
As Katara was still thinking about it, she wandered around her room to calm herself down when she suddenly noticed the vial for spirit water Pakku gave her on a small desk. She couldn't remember how it got there but she was all furious again remembering how she offered to heal Zukos scar and how she had to use it to save Aangs life because of Zukos betrayal. She felt like her feelings could burn her body down. She touched the necklace and suddenly the burning faded and she calmed down.
Is this what firebending feels like? The question suddenly popped into her mind.
She came to realize several things at once.
Zuko hadn't tricked her or meant to betray her, he was scared.
The reason why the rest of the Gaang accepted him was that they saw he really changed and forgave him and she hadn't. And the reason she couldn't forgive him wasn't for trying to hurt Aang or chasing them or teaming up with Azula in the cave but her very own selfish feelings. Back there she felt something she barely remembered... Being understood.