I think you may have got it the wrong way around: Stuttering, or perhaps rather more succinctly, the anxiousness involved in speaking, makes you tense up. You managing to get through it results in loosening, which means you've succesfully worked through your stutter. It wouldn't mean you managed to loosen up *in order to* speak without a stutter. I think it would make sense for you to work with a speech therapist to figure out if there are exercises you can do to counteract this bodily behavior of yours. :)