A Madman's Manuscript by Charles Dickens

This is Chapter 11 of Dickens' The Pickwick Club, and, fictionally, it is exactly that.

A man who, unbeknowst to anyone, becomes convinced he is insane, relates how he goes about the business of life keeping his dark secret.

I found it more gripping than Edgar Allen Poe's The Tell-Tale Heart and really liked when the narrator screams: "'Damn you,' said I, starting up, and rushing upon him; 'I killed her. I am a madman. Down with you. Blood, blood! I will have it!'

You can read A Madman's Manuscript by Charles Dickens online.