I read a book back in 2011, maybe 2012 at a stretch, that I’m pretty sure was YA. It features a female protagonist that, through some kind of accident or other similar event, ends up receiving an experimental procedure that accidentally extends her life indefinitely.
Her father may or may not have been involved in it. She is the only person who’s truly immortal, the world eventually gets access to whatever it is that made her that way but it’s altered to make sure no one else lives forever. I’m not sure if it was altered before or after it was given out to the world/released. No one else knows she’s immortal and she’s given another dose or shot or vial of her blood or something that she can use on another person to make them immortal as well whenever she finds someone she wants to spend eternity with.
She may or may not sacrifice the shot thing to save someone or for another reason, I’m not too sure. It’s a standalone book. I don’t remember any other details. If anyone could help find this book, I’d be so grateful!