I can't talk to a girl, help me!!!?

Laugh it up if you must, but I'm 21 and I am still afraid to talk to girls. For a short while when I was between 18 and 19 I was confident about talking to girtls, but I got rejected all of the time when asking them out, and I know for a fact that it's something I must be doing. But I don't know what it is, and I'm scared to talk to girls again because I don't know what it is. ANY advice at all would help. I'm too old to be in this phase for cryin' out loud, help me!!!