Hey Lynn1,
What you're going through is totally normal. The problem you're experiencing is a certain type of sexual performance anxiety...but specifically with verbally expressing yourself and being able to use words to titillate your man into a sexual frenzy.
The most important thing you must first realize is that if you're "in your head" (i.e. thinking about thinking and self monitoring your thoughts) then you are not connecting with your body. Thinking "what do I say?" and freezing like a deer in headlights is the affect this, and is counter productive to actually talking dirty the way you naturally can (and will).
Dropping all thought, all self criticism, and being solely in the moment is the opposite of this behavior. And having a few tricks up your sleeve can give you the confidence and courage necessary to really give it all you got (which is way more than you are giving yourself credit for).
Anyways, I hope this helps. Stop by my site and ask me any additional questions you may have about this stuff.
Stay Dirty,
Devian Day