What your experiencing is the typical experience all of us experience. It's not your fault and you aren't doing anything wrong or not trying hard enough. It might be the opposite of that. Trying so hard to control and not stutter is what is causing this problems. Finding safe places to practice letting yourself go for the feared word and giving your body a safe space to let it stutter on it is the better way I know. But it is not easy and you have to start in the easiest situations you can Ana build small step by small step.