Pretty much the opposite of mine. I stutter more when I know what I need to say. If I am working off the cuff I can get by fluently. Anxiety is a main contributor, but familiarity with a conversation partner does lead to more stuttering at times (family). If I think, "fuck em, I stutter" then I am either more fluent or don't care if I stutter because- fuck em, I stutter. Well, anyway, good on you for really thinking about it and reflecting about the causes. Knowing the reasons is a good start to changing your patterns or accepting who you are.