Here's a key: you have to make sure that she knows that you still love
her, and that the dress-up isn't because you are secretly interested in someone else. She probably fears that. No more advice than that from me, really; roleplaying doesn't do it for me.
EDIT: Ah, I remember where I heard something similar. I subscribe to the podcast
Fetish Flame. From their website, give a read to the article
To tell... or not to tell, and then for your SO's perspective,
Help, my partner has a fetish! Read them, and perhaps print them out for her.