I think the fears sum it up pretty well, mostly around the fact that you seem to be uncertain about where it will lead.
If it does lead on to some form of transition, then that's how it is. If you need that, fighting it will only cause you more grief in the end. I would see about getting to see a therapist or sex counsellor, a good one, and see if you can make any determination of how it is likely to be. At some point you have to tell her, dishonesty isn't going to help. It might be better to have a good gasp of this stuff before you talk to her, but from the sounds of it, she may be more understanding about it than you are.
