For me it's because when I'm alone or I read I don't think about what others think of me or my stutter, when talking to my friends I mostly talk fluently but to strangers and partially my parents I struggle a bit. I try to say my thoughts immediately so I can't think if there's a hard word or my stutter. This works pretty well because I can't take it back or think negative things, and if your thoughts are faster just accept it that one time and try it again later. Hesitation and changing words are a big reason why you have anxiety speaking my therapist says, she says my stutter is minor and soft but because I switch and change the hard words it's harder to understand and gives me even more anxiety. My advice is speak quick, think less and say it no matter what and if you can't one time that's okay, try again later it's fine to fail we speak everyday so you have more then enough opportunities to try again.