I'm sorry you were hurt by this behavior, I've also been on the receiving end and it's trash. I wasn't trying to excuse it, just explain it - I very much don't think understandable is the same thing as correct.
And also, again not to justify but b/c I think the author wrote this pretty textbook: Aya's really young and Yuu is in her POV the only decent person in her life (and I think she's not open w/ him and he doesn't push her to be) so when she's full on spiraling, there's nobody around to go "hey, you're spiraling, let's talk b/f you do something self-destructive"
Yeah, I think that's why Aya tells herself this is some noble act of sacrifice to save Yuu from herself. Like she knows it'll hurt him, she just doesn't think she's important enough for it to hurt him that much. Which again, not trying to justify this behavior, I agree that it is almost always selfish and comes from a place of self-preservation.