I was someone who would stutter alot and lose my concentration during day to day conversations with friends and family. Phone calls were a stuttering nightmare; and I always struggled with presentations and articulating myself confidently.
Fast forward, 5 years later I am taking phone calls as good as sales reps, and boldly delivering what I intend to say during my day to day interactions.
And the answer to how I did it is pretty simple too. Have more of those difficult conversations. Keep putting yourself in situations where you have to concentrate or articulate yourself in a given time frame.
Think of it as gym for your mind and vocal chords. Once you've had enough gains and exposure, you will feel no less than a bodybuilder, with the vocal boldness and strength of those who communicate naturally and fluently.
Even talking to yourself openly helps the cause. The key is to overcome your fears and not hold yourself back when you are communicating, and the fluency follows.