I have a really hard time writing anything other than dialogue. I've often started writing stories, and I realize that I have no description whatsoever. How do I stop doing that?
"Excuse me, can you tell me how to get to Carnegie Hall?" "Practice, practice, practice." Seriously, start by reading. Pay attention to how your favorite authors build their narrative. Then, see above.
Ditto what Cogito said. Also, try really hard to visualize the scene. Where are the characters having their conversation? What does it look like, sound like, how warm/cold is it, can you smell or feel anything? What are they doing while talking? What are they thinking that they're not saying? How might they reveal those thoughts through body language? What's happening in the greater world of the story?
Have you considered trying script writing, for stage or screen? Some people just aren't cut out to write prose, but if you're good with dialogue, your talents might be better used writing scripts.
So the character has some kind of problem writing descriptions; hmmm I see. She starts writing and then discovers she has no description whatsoever? It does look like an interesting premise. Doing what?
Aside from reading as the others say, I'd say play to your strengths. Some people (myslef included) struggle with dialogue - making it credible, or even extant - others struggle with the bits "inbetween" as you say. What immediately springs to mind is "Of Mice And Men" - it's snappy because of its reliance on dialogue. Use narrative sparingly and you'll perhaps write to your strengths in the same way.
You can try one thing: Try writing a story without any dialogue. Later when you re-draft it, you can replace some narratives with dialogues or insert dialogues between narratives wherever you feel dialogue is needed.
