This is a guess from me, but it seems applicable to many (if not all stutterers). You think about the words you are going to say. You think the words and that is completely wrong. Speech is supposed to be automatic, just as automatic as you breathe. You don't think about that!! You have an idea you want to communicate and that's it - the rest is done automatically by your brain. A lot can be said about this but you cannot say the words you are thinking. Practice to speak at the right moment when you get the urge to do it. Speak spontaneously. Just think about this. You are surely fluent sometimes. Think about what you did (or not did) when this happens. This was applicable to me not too many months ago. When I stopped thinking words, the blocks were gone. My speech happens automatically and I don't think more about speech than I think about breathing/how i breathe.