There was nothing that scared him more than the knowledge that there was nothing he could do. It wasn't that Peter was power-hungry. No, that wasn't it at all. He was simply terrified of being helpless. See, he wasn't afraid for himself. Sure, he didn't like being in danger. But what froze him in fear was knowing his siblings were threatened and there was nothing he could do about it.
It started with the bombing raids. He had gone after Edmund because he had to - he couldn't let his little brother get hurt. He had chewed him out afterward, not because he was angry, but because he was terrified of what could have happened. But Edmund didn't know that. And Edmund hated him for it.
Then it had been watching Edmund walk into the ice castle of the Witch. He had started after him, flying to save his brother, only for Mr. Beaver to tell him that he could do nothing. All they could do was walk away - leave his brother to the mercies of Jadis while they looked for Aslan. It tore him up inside.
At the river Peter had at least had a sword. But then the ice had broken and they had lost Lucy. If he thought his terror for his siblings was bad before... At that moment he was frozen, so stricken he literally couldn't think. Hi mind was completely blank. And then she had been alright. The ice in his mind thawed at the sound of her voice, and he wanted only to crush her in his arms and never let her go.
He had charged headfirst into the fight with the wolf, desperate to protect his sisters. But it had been alright. After all, he had a sword in his hand. There was something he could do. He could protect them, and he did. In some ways, that was the least terrible of many times that his siblings were in danger.
But the battle - the battle was the worst. All around him, Narnians were dying. Some faces he knew and had grown to love, and their deaths tore at him. Some faces he did not go, and he felt guilty that he didn't when they were risking their lives for him. But there was nothing he could do - just keep fighting, stay alive. And then Edmund had sacrificed himself to save Peter. Blinding, red hot rage and frozen, icy fear shot through him simultaneously. He couldn't save his brother - he knew that Edmund was dying. There was nothing he could do. He threw himself into battle with the Witch, wracked with sorrow, filled with anger. When Lucy healed Edmund with her cordial, Peter gathered his little brother into his arms and wept.
And that was just the beginning. All through their reign, his siblings managed to get themselves hurt or almost killed a multitude of times. There were often injured in battle. Several times Susan and Lucy were threatened by prospective suitors. Susan and Edmund were held captive in Calormen, and only just escaped before Rabadash married Susan by force. Their bodies weren't used to the sicknesses of Narnia, and each of his siblings was ill many times, sometimes even to the point of death.
then they returned to England, and Peter had to watch them deal with the hurt. He watched hope gradually leave Susan's eyes, saw Lucy grow sadder and more sober, and Edmund quieter and more withdrawn. He lashed out in anger - never, never against his siblings - because he felt helpless, powerless to protect them and give them what they needed.
When they returned to Narnia, he was desperate. They were back, but Narnia had changed. In his absence, terrible things had happened. He felt responsible, and powerless to fix things. He clashed with Caspian as he grasped frantically for his power, wanting to protect his family and people. He was glad to challenge and fight Miraz, feeling like he was finally able to actually do something. It was all his fault that things were terrible, he was convinced of this. If he fell to the usurper's blade, it was nothing more than he deserved.
Then Aslan had told him that he and Susan would never return. At once a huge weight of responsibility was lifted from his shoulders. Narnia was no longer his to guard and protect. It wasn't his job anymore. But at that same time - he was losing his home. He wouldn't have any power in England. There he was "just a kid". How would he protect his siblings? And then there was Susan. He knew what would happen to her, what she would choose. He had already seen it beginning, before. And Lucy and Edmund would return to Narnia without him. Who knew what kind of danger they would be in? Danger - and he would not be there to protect them from it.
When his little siblings returned from Narnia the final time, they wrote and told him every detail of their adventures. Reading their letters, he wept. So many times they had been in danger, so many times they had nearly died. How could he live with himself? It was his job to protect them, and he hadn't been there. He put his studies with Professor Diggory on hold, and went to his cousin's house to see his siblings. He had to see for himself that they were alright, that they were unharmed.
Physically, they were fine. Emotionally, they were a mess. Lucy wept into his chest for a good hour, because she would never return. Edmund had a broken, haunted look in his eyes. They were both to engrossed in their own pain and sorrow to see his. But Eustace saw. Eustace figured out what was going on with Peter. And somehow, Eustace understood.
Eustace managed to get Peter alone on the third day of his visit. They talked of many things, and Peter was staggered by the amount of wisdom his formerly obnoxious cousin showed. And by the end of the conversation, Peter felt just a little bit better. How Eustace managed to knock some sense into him, he wasn't sure. He would forever be afraid when his siblings were in danger, forever be afraid of being powerless to protect them. But at the same time, he understood that what happened on the Dawn Treader was not his responsibility. Aslan had sent his siblings back without him, he wasn't guilty for any hurt they received. It wasn't much, but it was a start on the road to healing.
No one knew just how broken Peter really was. He was the strong one, the one who had it together. The one that all the others turned to - even Susan, to an extent, after she chose to ignore the past. Even Eustace, though he recognized his cousin's pain, did not know just how deep it ran. But Aslan knew. He waited patiently, knowing that soon it would be over. Soon High King Peter would once more be king in more than name. Soon he would come to the Lion, and be fully healed.
Peter saw the train careen out of control, and momentary panic filled his heart. Some of his family was on that train - the rest was beside him, in the train's path. He slung a protective arm about his closest sibling - Edmund, it turned out - as the train crashed directly into the platform. when he opened his eyes, gentle golden light met his gaze, and gentle peace filled his heart.