Talking to yourself and to others are completely different for me, the former being 100% fluent. Talking to family I am more excited and maybe make less of an effort than when I am with a stranger. It isn't only stress that exacerbates stammering for me. For example being happy can make it worse. Some books I read suggested it is a subconscious/physical thing which is reinforced by fear, in a vicious circle, which makes sense to me. However the brain/body is so complex there could be multiple causes.