@crynelleciaz
To Joonwoo, he learned that the "very very solid relationship" that he built from the start was just what Sungah was showing in public, while hiding feelings of fear and disgust. He doubted everything that Sungah ever said positively about him, even when it was said to her RanChat match. To him Sungah only said that Joonwoo seemed nice to her RanChat match because her RanChat match was the one that suggested that she try befriending him, while the whole time she must have thought he was a creep, but only opened up about her true feelings through the app when they met up at the beach.
I think that he looks back at the months he tried to get Sungah to like him vs the one week he knew Lila in terms of how they responded to him. While Lila is clearly gas lighting everyone, she frames it in a way that feeds Joonwoo's self esteem issues. He knows that he can't do anything about the past, but thinks he can still help her now, the way she helped him. Her feelings seemed genuine, though twisted. So he feels like he could have made an impact on her life. When it comes to Sungah, he thinks of months of lies and wasted efforts.