Anyone with a minimum of intellectual honesty will accept that this is only partly true. Any circumstance that you find yourself in is a combination of many factors, it can never be naively boiled down to the single factor of "you". There are many variables that go into putting you, the reader, and myself, in the exact situations we are in now. That our actions have influenced it is beyond discussion, but that it has the potential to uniquely shape it, regardless of external factors, is absurd.
So, you may say: "Sure, external factors will affect you, but where you go from here is completely up to you" -- but this apparent truism does not hold up to scrutiny. If your life so far could not have been shaped exclusively and purely by your own initiative, how can the rest of your life?
The church of individual performance and the cult of myself is so intimately tied into our current zeitgeist or hegemony that nobody even seems to question it. I really wish more people did, if nothing else it would make for more interesting philosophy, possibly something that felt a little less like "The Secret".