It might be a mix of both, I'm no expert. But I know when I'm relaxed my fluency is definitely up there, but when I'm nervous or frustrated it definitely rears its unpleasant head, and that in itself is both physical and mental. Try deep breathing exercises? I practice them everyday, it helps me cope so much better. Also check out my thread on here for how I work through my own problem, I can elaborate further if you'd like. Where there is a will there a way. Look at the likes of Samuel l Jackson! One of the most famous people with our same issue..