Like the other poster said, it has a lot to do with confidence in self. When it came to those affirmations they came from my own beliefs about myself and insecurities. Challenging this beliefs and continuing to tell myself the opposite of those until you begin to see yourself in a different way. When it came to the stutter I would tell myself that something similar to what you wrote, “I’m a fluent speaker”. I found that affirmation like that were harder to “set in”. So, I changed it to something like to a more accepting statement, “you are not your stutter”, “it’s ok to have slips”. Once my confidence in like rose my stutter lessened. This by no means is a magic way of “curing” speech fluency and took time but, how I think about myself definitely changed my speech patterns.