My Alex is a non-smoker because of a bad experience the last time she tried smoking. That's something that happened in the past, but it didn't become true until I declined Jonas's offer of a light. My decision in the present altered Alex's past.

Ooh, I like that! And I want some kind of narrative where the character becomes increasingly aware that they're a former blank slate who is becoming increasingly more specific because of someone else's decision, and freaking out about it. And not like "They keep making decisions that make you feel like you have specific memories", but "They keep rewriting reality so things that they decide retroactively always have happened." Especially if the character's reality is being rewritten not by a malevolent figure, but by a person who's been accidentally handed the power somehow and initially doesn't realize it, but discovers and also freaks out, but doesn't have a consequence-free "stop deciding things" option. (There would be a scary, possibly-malevolent entity in there somewhere.)

That's fascinatingly complicated about the reflection!