I am quoting from https://www.inc.com/sims-wyeth/how-to-slow-down-if-you-talk-too-fast.html but you should read the full article.
"So here is an exercise that will cure you of your malady. It was given to me by Marian Rich, a voice and speech teacher in New York who worked with many famous actors to help them improve their vocal presence. The exercise will teach you that your voice is a wind instrument, and you must have ample air in your lungs to play it well.
Mark a paragraph / in this manner / into the shortest possible phrases. / First, / whisper it / with energetic lips, / breathing / at all the breath marks. / Then. / speak it / in the same way. / Do this / with a different paragraph / everyday. / Keep your hand / on your abdomen / to make sure / it moves out / when you breathe in / and moves in / when you speak.
Before you whisper each phrase, take a full bellyful of air and then pour all the air into that one phrase. Keep your throat open, and don't grind your vocal chords. Lift your whisper over your throat. Pause between phrases. Relax. Then, take another full breath and whisper the next phrase. Whisper as if you were trying to reach the back of the room.
Once you've whispered the paragraph, then go back to the start and speak it in a conversational way, but again, pour all the air into each phrase and honor the silence between phrases. I can't stress that enough. Take your own sweet time at the forward slashes.
Also, take deep pleasure in enunciating each resonant vowel and delicious consonant. Give your lips and tongue the assignment of shaping every lovely syllable.
I like to do this exercise as though I were standing on second base in Yankee Stadium. I do it like an old-fashioned orator. I raise my arms up to address the crowd, speak in a loud voice, and pretend I have to spoon out each phrase very slowly because there are 60,000 people in the stands, and my voice has to travel a long way to reach their ears.
And please, don't misunderstand me. I am not suggesting you actually deliver presentations pausing between each phrase. Rather, I am suggesting that you use this exercise as a tool to teach your mind and body how to slow the heck down.
Repetition is key. I bet that if you do this once a day for 21 straight days you will cure yourself of speaking too fast. Let me know if it works."