Growing up I had a quite severe stammer. I went to a speech therapist for three years and after she found something that worked and also just with time my stammering disappeared completely. Three years later my stammering came back with a vengeance. Went back to the same therapist, nothing worked. That's been fourteen years ago now, and I've been stammering ever since. It does fluctuate over the years, however. My former speech therapist and quite some armchair experts think stammering/stuttering is related to how you feel during different periods of your life. The explanation being, if you're constantly stressed or faced with uncertainty you stutter more and when you're feeling confident you stutter less. I don't think that's true. For me it doesn't hold true at least. Stuttering makes me nervous, but being nervous doesn't make me stutter.