So.....she was the one who broke things off? Just to get this last bout of freedom out of herself? So she can take care of the mother she "broke" out of pure guilt?
Then, if she was the one who broke things off, why initiate the kiss with the MC if you were "done" with him? (before the flashback)
Again, I appreciate more context with the character, but it's leaving with more hanging questions now.
She broke things off with him because her family situation went from "seriously fucked up" to "completely fucked up," and she didn't want to drag him into it.
While that might feel cliche, she's very much acting like a certain type of child from an abusive household, where everything is their fault and they'll just accept blame and take responsibility for everything. She's been told all this is her fault, so all of this is her fault. She doesn't deserve to be happy, and she shouldn't drag anyone else down with her. All she'll ever be is the failure of a daughter who broke apart her mother's marriage and ruined her life.
I imagine that Yuu asked her what the hell happened, and it went from there. Cause she didn't STOP loving him, and the last thing she said to him sounded like she was fucking dying of cancer, telling him to have fun, to fall in love. He probably didn't forget her. How COULD he? They had a serious thing going. They broke it off because she didn't want to tie him down, something he wouldn't have accepted if he knew. Considering we get him going "but" and her going "Shut up! Who do you think you are," and them stepping closer and kissing, I can easily see him pushing the subject and her feelings overwhelming her. Because he WAS the only light in her life, the only place she felt she belonged. She willfully left him because she didn't want to drag him down. Him even broaching the subject of "I think that's a bad thing for you to do to yourself" would probably cause a cascade failure of emotional barriers.