I've had a name for my alternate female persona ever since high school, even though I'm just now getting around to transitioning some 12 years later.
My birth name is Charlie, and I ALWAYS wrote transgender fiction on my computer to try and live out the dream, if only in fantasy worlds. So pretty much as soon as I started writing these stories, I started finding female versions of my current name. Cheryl was the first one that came to me, but I've never liked that name, so that didn't last long. And then the next one that I thought of was Carrie. And I don't know what it was about that name, but it just seemed to fit my personality so perfectly for some reason. And so ever since then, my female name has been Carrie. And even though I've discovered many more female versions of the name "Charlie" in the years since then (most notably Carly and Charlotte,) none of them feel like me as much as the name "Carrie."
I asked my mom a long time ago what my name would have been if I was born a girl, and it would have been "Sarah Elizabeth," named after my two grandmothers, and Mom says she would have called me "Liz" because she likes that name. So once I (hopefully) get to the point where I want a formal name change, I'm going to combine the two, and hopefully be Carrie Elizabeth [last name withheld.] I think it fits, especially since my mom always tells me that I have her mother's eyes. So taking on her name as a middle name really seems right.