That knife won’t do. Nor will chains, whips, or swords. An exception will be made for blades if you put them in the fire beforehand. You must think of this as a cauterization. Only fire will do.
You know exactly how long it has been, you needn’t add another chalk mark to the far wall. You know you ”forgot” to put a mark there eleven days ago.
You must think of it as an open wound and of yourself as someone who has been bleeding too long. It will leave a scar. He is the open wound. This is critical.