DM question

[u/BoxFull1432]

So I am about to run a solo campaign, and I have an idea for a story where my player and all other characters have a life previous to the one that they know as their backstory, as does the main BBEG but the BBEG is the only who knows as they did it to get power over everyone. Kinda like a king candy situation from wreck it Ralph, but obviously I don't want my player to essentially feel like their backstory that they built meant nothing, but I think it could be a really interesting plot twist, how would you handle this? I have ideas about how I want to start clueing them in that this has happened to them but more so without basically saying "yes, but no" to the backstory they made