When people try to slow down they make the mistake of thinking it means say each word individually. While this is slowing down, it also creates more chances for us to stammer. It's almost like starting a new sentence every word, which you know how the beginning of the sentence can be the hardest part. Avoid that. Instead, think of it as talking in slow motion. Picturing someone running in slow motion or your favorite slow motion scene from a movie is a great mental example of how you should talk. Not word by word, but one cohesive sentence starting on the exhale. You may think it sounds weird at first, but done at the right speed sounds normal to other people. Another way to slow down is to make sure that you are articulating every word completely and clearly. Clearing pronouncing every prefix and suffix will not only improve your clarity, but it will also force you to slow down. Lastly, I personally do breath work for 30 minutes every mornin. This will strengthen your speech all around.