True, you may not be able to stop crossdressing. But you do have control of when and for what interval you do it, and to what degree she has to see it. Just as marijuana was feared to be a threshold to heroin and cocaine, crossdressing may be feared to be threshold to becoming gay or transsexual. You need to reassure her on both counts that you aren't. And you can't do this with just words, the suspician may linger on.
Lies are bad for relationships.
If you tell one lie, you have to tell ten more to cover it up. You'll also have to keep lies straight as contradictions get noticed. Crossdressers pride themselves on how they understand women better by having walked in their shoes. Now's the time to put it to the test by really understanding a woman's point of view in the relationship and what she wants. Start developing new interests.