"I know you deserve someone better..."
That's a smoking gun right there, if anybody still had any doubts that Shia had low self-esteem and was insecure about herself.
Same, 2 deep for my autistic brain, but it seems to be progress?
I think?
So Tasuku finally realized the disconnect between their motivations to watch movies together.
He was only watching movies with her because he wanted to get closer, but she wanted to watch movies genuinely, as a hobby, with him. Because he previously wasn't really watching for the sake of enjoying the movie with her, her desire to be movie buddies with him is left unfulfilled and that makes her unhappy.
That allowed him to connect the dots with Shia's hurtful previous statement: she realized it before he did and tried to brush him off... because she was very frustrated — that he was clearly only watching movies with her as a means to act on his crush — and not out of a genuine desire to enjoy movies, with her.
He thinks he's the worst because he's in fact, been doing her a disservice all this time, until he started to enjoy movies himself.