Severe [sometimes] Stammering! Any advice?

Severe [sometimes] Stammering! Any advice? Hey all! My question is what technique (like Yoga, Mediation, breathing exercise ) is useful for controlling my stammering. Sometimes I stammer very badly and sometimes not. Earlier, I had tried Meditation​ (on the recommendation) but quit it after a week. My problem is before any speaking session (where I suppose to speak), I constantly think about my speech, how I'm going to speak there, and just before the session my heart beat increases. And when the time come to speak I barely able to speak clearly. Any pratical advice? Anyone want to help me to teach those technique?