I know exactly what you're talking about. I don't know the nature \ cause of your stutter, but i think I understand why I do what you described. Hopefully this will help! Whenever im talking with friends or family, im more relaxed and focusing less on my individual words, and more of what im saying as a whole, so i stutter slightly more than usual. But in a situation like giving a presentation, i focus on the specific words more, so i catch myself stuttering before i do. (Idk if that makes sense). Memorization and preparation might also be a factor, since you normally don't prepare scripts for casual conversations. Again, your stutter is more than likely different than mine, so I don't know how much this will help, but i sincerely hope it does in some way!