True, it's mostly in our minds. Negative experiences contribute to make the point of view we have on stuttering worse, and we end up worrying about it which results in anxiety and so on. You can see you stutter less when you just let go and don't think about what you have to say. Every stutterer has his own "cure" since every stutter is different in some way, and we have different ways to deal with it. Placebo effect probably helps too, since it can help with confidence.