A boxing analogy you may wish to consider
A very common problem. Everyone goes through something like this at one time or another.
Like almost anything else, practice strengthens the fundamentals. But how to practice if you can't get out of the starting gate?
Ali, Spinks, Holmes, Tyson et al didn't start out by leaping into the ring for a heavyweight match. They practiced and practiced and practiced. They worked on the fundamentals - strength, speed, skill - before they even got into the ring.
Cultivate a "sparring partner", one who's supportive of your efforts, that you can practice with. Preferably a girl, though it doesn't have to be. Simply start initiating a conversation with your sparring partner, then after a few minutes stop and go over the results with him/her. How did it make them feel? How did it make you feel? What worked, what didn't? Then try again.
Soon enough you'll have the confidence in your own abilities to initiate a conversation with a complete stranger. The first time will be difficult, and it probably won't go well (then again, maybe it will). Just relax, regroup and try again.