I do all the exact same things as you, especially not finishing someone's sentence but for some reason I get really anxious. My stutter isn't always very bad, some days I might not struggle to speak at all and other days every sentence is hard work. I don't really know why but I worry that if I stutter when talking to someone who has it worse than me they'll think I'm taking the piss. Ive never once thought that about someone else when they stutter though, something for me to work on for sure