I know Shunsuke isn't confident in himself but he should care more about Kiyoka's feelings. Like, if you love someone, you should think of what will make her happy first, instead of being too self-centered and staying away from her on your own.
Because when you have absolutely no self confidence, the idea what she was attracted to him wasn't ever a possibility. He feels disgusted at himself for being attracted to her in her Summer Uniform, it's a very painful kind of self hatred.
Even his Relationships feel one sided from his perspective. He is expecting to confess and then get rejected and accepted that as inevitability. I've been there, hell I am there right now. He sees himself as a Toxic cloud of poison that only makes those around him worse. He can't imagine that people enjoy spending time with him or even want to spend time with him. By avoiding her, it can only make her life better. He's played out that scenario in his mid probably a 1000x times, imagining what she's thinking and how she'd respond. It's a combinations of Cognitive distortions and as she said, you can't read someones mind.
Hopefully this helps him grow