Peter was depressed and anxious. He always felt like the hanger-on, like he wasn’t good enough to be friends with the rest. James was the jock, the best in their year at Quiddich with a body that made all the girls track him with their eyes. Sirius was the best looking boy in their year, carelessly talented in a way that made Peter’s insides twist to see it. Remus was the smartest – or the hardest worker, it was hard to say. In any event, he had the best marks in their year, and that was with missing classes every month for the moon. When Peter looked around at his friends, his chest swelled with pride while his stomach knotted with fear they would find him out.
When Peter didn’t want to get out of bed for a solid month their first winter, it was James who brought him breakfast and tried to coax him out of bed with promises of a workout together. Sometimes it worked. Other times it didn’t, and Sirius would bring him lunch and tell him about the pranks they were planning and the girls Peter had an eye on. Sometimes it works. Other times it didn’t, and Remus would bring him dinner, and notes from all the classes he’d missed, and read to him. And sometimes this worked, and Peter could get up. And sometimes, he felt even more wretched for making his friends spend so much time on him.
When the depression waned and he was able to live life normally again, the anxiety would kick in. He’d make pages-long to do lists, things that he wanted to do for himself and for his roommates, to thank them for being there for him, and to try to make it up to them. He organized all of his things to within an inch of their lives, to try to make it easier to keep up with when the depression came back. He rewrote all the notes Remus took for him, with footnotes and copies of relevant textbook pages, for himself and for Remus. He helped Sirius with every prank and worked out with James whenever he was asked. Every time, he was certain this would be the time they began to distance themselves from him, and every time he was determined to be the perfect friend they deserved so they wouldn’t.
Once they graduated, it became harder. They no longer lived together, but they still tried to check up on each other. They spent time together, on and between missions, but it wasn’t the same. Everyone had missions and everyone was in danger all the time. Peter wasn’t a good deuller, so he poured his anxious energy into being useful in other ways. Organizing the intelligence they received so it could be better used. Learning healing spells and potions to help the injured who came back after every mission. Cooking and cleaning at HQ so nobody else had to worry about it. But without his friends there to support him and encourage him, he began to feel taken advantage of. He began to believe the parts of his brain that told him they didn’t really love him, that he wasn’t good enough for them, and that they were simply taking pity on him. He began to feel like they only kept him around for what he could do for them, rather than loving him for himself. He began to feel like he had been right all along, and that they’d finally realized that he wasn’t worth it.
When Voldemort finally got ahold of him, those were the thoughts racing in Peter Pettigrew’s mind. The absolute conviction that his friends hated him. The bone-deep knowledge that none of them loved him, or cared for him, but they were too used to his service – or pitied him too much – to push him aside. The nagging suspicion that they simply didn’t think enough of him to push him aside, and that they wouldn’t even notice if he failed to show up one day. Voldemort could see those thoughts in Peter’s mind, and recognized the chink in the Potter’s armour that he’d been looking for. Voldemort didn’t just threaten Peter with death – he mocked Peter with the idea that Peter would die for people who didn’t care for him, who had never cared for him. He reminded Peter of all the days he had spent, practically catatonic, while his friends were out on missions. Of every time one of them had been short with him, or seemed distracted during a conversation, of each time there was tension he wasn’t able to resolve or favors he forgot to do. He convinced Peter that his depression was right, and nothing mattered. He convinced Peter that anxiety was right, and his friends barely tolerated him. And then Voldemort mocked Peter for being willing to die for those friends.
And that is why Peter broke.