Wow thank you so much for putting it into words...
For me, they are having relationship is far from ideal but it gave them some kind of hope in their painful life. I agree that they need more therapy than just go straight to dating, and I guess that's why the authors are being careful to show the progress in their relationship? Like there's no happy moments (like the Christmas date) or intercourse so far. Just them threading the new territory: anxious, but hopeful.
I think with the way authors draws the ending so far, they wanted to leave lots of room for our imagination, the extent of the therapy included.
As in I completely missed a major theme of the story and just how clearly Dohyun and Sungho are each other's foils lol. So I feel like the main theme of the story is how to cope with the transience of all relationships. The high school relationships of the main characters pushed them in two opposite and unhealthy directions: Sungho clinged obsessively to his memories while also slightly distorting them to place more blame onto Dohyun (he actually admits to himself that Dohyun didn't really pressure him into sex at one point, but he holds onto the idea and his hatred of Dohyun due to internalized homophobia.) His feelings quickly spiralled into a terrifyingly severe mental illness which ultimately contributed to his crimes.
Dohyun meanwhile went down a more functional but ultimately extremely damaging path. From early on, he came to believe that all attachment to others results in pain. When he was together with Sungho he allowed himself to hope that things might be different, and that his duelling hope and doubt led him to test Sungho's attachment to him, which in turn made Sungho desperate, which in turn led to tragedy. A sad case of miscommunication.
In any case, the way things ended cemented Dohyun's mindset: his chief goal in life is to avoid pain, which is why he'll take pleasure when it falls on his path, but never pursues anything. No ambition. Nothing to look forward. No-one he cares about or who cares about him, all in the name to avoid the pain that would come when the relationships would inevitably end. He's functional, but a pretty shitty person and definitely unhappy. It's all survival all the time, which is why he so easily forgot everything about his HS school that might have caused him pain. And what did all those sacrifices to avoid pain get him? A dead-end life, followed by kidnapping and awful abuse with no end in sight. Pain upon pain upon pain. No wonder he cracked.
In this light, I feel a little better about the ending: it's two people with horrible, (self-)destructive approaches to relationships finding some kind of middle ground: actually having a meaningful connection with someone (Dohyun) without ruining everything with too much possessiveness and obsessiveness (Sungho). So at least it makes thematic sense even if I'm screaming THERAPY NOT DATING at the screen. With that said, I still wish Sungho had answered for his crimes as well as gotten some SERIOUS psychological help. Dohyun definitely needs serious help too is symptoms during his breakdown match with acute stress disorder which can turn into ptsd if not treated :( But at least as a short-term thing, maybe them being together gives them a better chance to actually improve their lives. I hope.
(yeah I do think they were actually getting some kinda therapy at the end but i'm not sure that's enough. :/ Fingers crossed.)