This particular scene is from Aang's POV at the end of Book Two right before Azula shoots him with lightning.
Aang
"I'm sorry, Katara." I say as I know what I have to do. I have to let her go. Maybe not forever, I don't know. Once I win the war, I won't need to go into the Avatar State at will, and I can love her like this again. But even as I think it, I know it can't be true. I can love again. I can love another after this war. But right now, I'm deciding something else is bigger and more important than how I feel for her. And I know I can't come back from that, because even if we win the war, if anything ever happens again, I'll know I made this decision before and I can make it again. I can always put her second to the world. And knowing that, I know I can never be with her again. Because I know she deserves someone who can love her adn save the world, but if he couldn't, he'd let the world burn before he let go of his love for her.
I take a deep breath and release it, and go into the Avatar State, but this time I'm in control. I rise above all the Di Lee agents, Azula, and Zuko, about to strike. But suddenly I feel an indescribable amount of electrocuting, excruciating pain. And I feel my connection to the spirit world fail. And then i'm falling, with no strength to stop. I can feel my spirit almost leave my body. If it does, I know that this will be the last trip to the spirit world I make. I'll stay there. The avatar cycle will end. But I also feel Katara catch me. I don't know how much time passes before I feel her hands on my back, healing the burn. It anchors my spirit to my body. I know I'm not in danger of dying anymore, but it still takes all of my strength to open my eyes. When I do, I see her smiling down at me. I weakly smile back. I love her. But not the way I did. Avatar State or no, I let her go.