It is a common problem these days that many people are facing.
You may think its a long challenging task but it is quite achievable. All it requires is some consciousness and thinking and you can easily overcome your problem.
Next Steps
when you are taking to someone, take a deep breath between each sentence and frame your thoughts before you speak them out loud.
Ask your close friends and family to remind you to speak slowly when you are speaking fast.
Stand in front of a mirror and practice speaking, observe yourself as much as possible.
If possible, speak in front of/interact with large grp of people.
Health Tips
You can also try homoeopathic medicines together with above mentioned techniques to achieve better results.