Email is way too impersonal for something as important as revealing ones passion for crossdressing, especially to a significant other. If you can't find words, or get nervous over a person to person talk, perhaps a script, tactfully written in your own words would be effective. Of course you would have to memorize the script so it comes from you verbally. Try rehearsing in front of a mirror to gain confidence then you may be ready to tell her in person.