The girl who can't talk dirty. . . oh, wait, that's me!?

It's personal, I know. I've alo realized that this is a good way to get help, though, so I don't very well mind. Anyway, it's as it says. I can't talk dirty in bed. I always feel stupid and get embarred, which makes me completely clam up. My current boyfriend has mentioned it more than once that I can't, and he does try to get me to in little noticeable nudges, but they don't work. I said something one time, and I've been mortified of it since, and hence never tried again. He doesn't press me, but I'd like to not make him the only vocal one in bed. Can anyone give me any advice on how to talk dirty maybe?