Advice needed for projecting voice and consistent, clear speech.

Advice needed for projecting voice and consistent, clear speech. I guess this is the most appropriate place to post this thread. I'm 27 years old and all my life I've been introverted and at times extraordinarily shy. I just don't seem able to put together a consistent flow of conversation without mumbling, slurring words or stuttering when talking to anyone, including close friends and family. I seem to be at my worst when talking to someone in public, I feel like the strangers around me are listening and analysing every word I say. This causes me to lose any sort of projection in my voice and whoever I'm talking to asks me to repeat every second sentence I say. Everytime someone asks me to repeat what I said my confidence drops and drops. I would appreciate any advice.