Instead of putting 100% of your focus on slowing down, think about putting your 80% on fully pronouncing your words and 20% slowing down. You will slow down by default. Also, slowing down doesn't mean talk in slow-mo. It is perfectly okay to take pauses in your speech. Taking 1-3 seconds mid sentence to think about what you want to say feels more fluent than you think. Go watch Obama give a speech and watch his pauses and cadence. (Pro tip. After he says a sentence. Pause the video and repeat the sentence in his cadence so you can really feel the difference.) This practice takes 5 minutes and you can learn a lot. Please let me know how this works for you.