In search of a story

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So sorry to bother you all with this but this is driving me nuts and my mental health is kind of important to me.
I am searching for a story, rather short but with the brilliant idea that Bertie's mind (his expectations and wishes) changes the reality he lives in. As usual Jeeves knows all and Bertie and everybody else is clueless. The story is basically aunt Agatha scaring Bertie about the oncoming war and Jeeves alleviating his fears in the hopes that then it won't happen if Bertie believes it strongly enough. Does anyone know this? I would be so very grateful!