I have a book to recommend! Her Voice is a Backwards Record, the best novella based on a philosophy paper I've read.
If all physically possible universes exist, so do all physically possible girlfriends… including the one you're imagining who's imagining you.
Ana's parents, who hadn't heard that it wasn't game-theoretically optimal to punish her for dropping LSD, packed her away to a troubled teen camp. During the day, Ana faces the harsh Utah desert, near-starvation, and torture in the name of "therapy." At night, she seeks desperate comfort in the arms of her alternate-universe girlfriend, Yuya, one of three hundred wives in the harem of the Emperor of Every World. But as their respective prisons wear away at their very selves, both girls face a choice: to become the monsters those in authority want them to be, or to die trying to escape.
Caveats:
- I do personally know the author and helped beta this. I wouldn't recommend something I didn't like just because someone I knew wrote it, but I will admit to being biased
- there is torture and institutional abuse in this one. It's Heavy.
- on a lighter note, Ana and Yuya do have explicit multiversarial sex while 17, so that's a heads up for that if you want it.