Life.
Beca hated the word. She really did. She didn't like the way it sounded when you promised someone your life, but gave up. Really, she didn't have much of a choice in giving up. It wasn't for lack of trying. She tried to keep the relationship alive - healthy - despite the efforts of destruction that were thrown her way. She was determined, but a person can only fight for so long.
She gave up, and moved on. Every single day she woke up and hated herself for that, but she would never let anyone know, especially not her wife. It was her own private hell to deal with. No one would understand why she blamed herself, because she had tried so hard to save it.
In the end, life fucks you over.