Hi! I think I can understand why you questioned your sexuality, after all, if you enjoyed sleeping with Cindy all these years (line of thinking)....however, I would say that because you perceived Cindy to be a man (for obvious reasons!) that wouldn't make you a lesbian. If you are actively attracted to women in general, then you could be a lesbian, but simply because your partner describes herself as a lesbian doesn't automatically make you one. If you're making an special exception for her, it doesn't make you a lesbian, either(unless you want to consider yourself that way, it's really up to you on that!)
I, myself, am bisexual to begin with so the sexuality issue hasn't been too hard on me, but I do sympathize with you! It isn't easy to think that you're sexuality might be something other than what you thought it was and I can definately identify with that! It took me ten years to fully realize that I was bisexual and not "heteroflexible" as I called it (this meant that I didn't mind sleeping with a woman, but that I didn't want a romantic relationship with one, and it was erroneous thinking anyways!)
Enough about me, though. I can tell you that worrying about being the cause of this is nonsense and you shouldn't believe it for one second if pops into your head again. Just clearing that up for you!

It's tempting and somewhat comforting to lay the blame on something or someone for all the turmoil you're experiencing, but there really is nobody at fault for this condition (and nothing that's been scientifically proven as a cause, either!) Besides, blame leads to guilt and guilt isn't helpful.
Honest and open communication is what you need. My only caution is that you both need to be as objective and open-minded about what the other is saying and really listen to what is being said. Which I know is a lot more easier said than done, but if it can be done it will work wonders on figuring out where to go from there. Take baby steps, if you can manage it. Best of luck to you!