Time Already Spent Is Not a Reason to Keep Going

You have read a hundred pages of a book you do not enjoy, so you keep reading — after all, look how much you already invested. This is the sunk cost fallacy applied to time: the hours you already spent are gone no matter what you decide next. They cannot be recovered by finishing something that no longer serves you.

The only question worth asking is what does continuing cost me from here? Judge a project, a class, or a relationship by what it offers going forward, not by how much you already paid for it.