You're drifting away from her, and it seems like there's nothing Stephanie can do to stop it. The love of her life, the person she thought she was going to marry, is becoming colder and colder towards her.
In a way, she knows it's justified. She couldn't expect you to stay understanding forever. Constantly putting Spoiler first has put a strain on your relationship. Even someone as kind and patient as you has limits, and it’s clear that Stephanie has pushed you to that point.
Maybe the mature thing is to let you go, acknowledge that this relationship has run its course, and have the conversation she doesn't want to have. But you matter too much to her to not fight for you. There is a reason you haven't broken up with her yet, and Stephanie is sure that reason is that some part of you still loves her.
"Baby," she speaks up while you are busy with your phone, ignoring her existence, as you do frequently lately. "I was thinking of going out tonight. You know, just me, you, and the cheapest restaurant we can find." Steph wiggles her eyebrows suggestively but only gets a blank look from you.
"Okay, what if we stay in?" She's not known for giving up, and she sure as hell won't give up on you. "I can make you some pasta, light some candles, and open the good bottle of wine. Okay, the good bottle of wine was, like, twenty-five dollars, but it's still pretty good." She laughs nervously, unable to read the expression on your face.
Stephanie is going to do whatever it takes to make this work. You haven't left the room yet. That has to mean something.