As many people have pointed out here, you are experiencing a block. The speech mechanism is so delicate that the slightest stress can cause it to go berserk. Stutterers are already predisposed to having slight abnormalities in their speech (called the 'primary symptoms' of stuttering; for example, b-b-basketball). What happens as stutterers get older is that they get more self-conscious, and feelings of shame and embarrassment caused due to stuttering in social situations result in them developing 'secondary symptoms' (like a long pause, facial contortions, jerking one's limbs etc.). If stuttering is not treated at all, these symptoms (which arose so that the stutterer could avoid/postpone stuttering) make the stutter worse and disrupt communication. Now, the good news is that in most cases, secondary symptoms can be reduced to the extent that they are virtually invisible. This way, a stutterer can learn to communicate effectively, despite having a mild stutter. The first thing you have to realize before deciding to improve your speech is that it needs constant, conscious effort from your side and will take time to improve. A two pronged approach has worked for me, and I hope it works for you. The first thing you have to do is analyse what you do during stuttering that is unnecessary for speech production (like jerking your hands, shutting your left eye, tightening your jaw muscles etc.). You have to consciously try to modify these behaviors and replace them with the correct ones. Second, you have to face your fear of situations, and gradually use the techniques you learnt in the previous part in increasingly difficult situations. This is easier said than done, but trust me, you will not regret it. Slowly, you will begin gaining confidence in your speech and be able to socialize and lead what most people would call a normal life. I hope this helps.