This was my huge fear about 3 months ago, and my big issue I wanted to address starting therapy. I thought I was convincing myself of something that wasn't true. I finally bought some clothes and makeup, took a look in the mirror and could say (with a huge smile on my face) "Oh, nope... this is definitely real". I don't know that there is an easy answer to finding peace of mind with this issue, unfortunately. I feel like it may be something that you have to find your answer for organically, be it in therapy or just by doing something like spending time dressed, or whatever you think may be your answer.
But, the good news is, you aren't alone in those fears by any means!