She doesn't want him back because Hikari has a crush on him, she never stopped loving him. She ghosted him because in her head it would be easier for him that way and he could move on with his dumb, happy, simple, pure life without her. She was devastated when she turned her phone back on and he hadn't. And the stated reason for even leaving him at all was to take care of her insane, suicidal, hospitalized mother, who she feels responsible for making. The unstated and implied reasoning further being as a kind of ritual self-flagellation/self-isolation over that guilt and her sense of being too tainted a soul to stand by him and bring him down when he was already obsessed with (who turned out to be) Hikari.
Yeah, it's dumb, but it's absolutely miserable, depressed teen logic that checks out.
The thing I assume she means isn't fair isn't "I can't be with him," or "she can," it's that a shameful and heart-rending (for her) and traumatic memory in her time with Yuu can no longer stay buried and will soon come bursting back into her life. When? Dunno. Soon. Even if she had made peace with that, it's also now unavoidable for her that through very unlucky chance, some really shitty things she did in the past have the potential NOW to badly hurt her best friend, after same friend helped her reform from being as much of a self-destructive jerk.
And yeah, the only two people she's felt really close to are destined to be together, in her mind, and when they finally hook up, she's going to be all alone again at best, and hated by them at worst. The fact that karma's come a-knockin' isn't unfair, but to Ayami, the punishment feels a little disproportionate.
Actual spoiler theory:
Little does she know that it isn't going to play out like that at all, and when she accidentally meets up with Yuu again after trying to avoid him at the festival, he isn't mad but instead tells her he's never for a minute stopped loving her since they separated, hence the ol' slap slap kiss kiss.
edit: ugh, it would make sense if she hit him because he kissed her first, then her willpower crumbled.