How can I use the idiom 米已成炊 to explain "once I've told you, I can't un-tell you"?
米已成炊 or 木已成舟 can't be used like the way you use them. They describe a situation where you can do nothing but accept the fact because something has already happened and there's no way you could undo it. 

Usually, it goes like: 


  我已经把我的故事都告诉你了，没有办法再收回来了。 现在米已成炊了，我能怎么办呢？


Or 


  我已经把我的故事都告诉你了，现在米已成炊, 没有办法再收回来了。