Rather than not being over him, its more like he is still traumatized from what his ex did, he is afraid how people can change for worse.
That feeling when you thought you knew that person and for someone you trusted so much just to act like the opposite of what you expected, just like he said.. he was naive, he didnt expect him to change like that.
Yes he did took advantage of Eden's presence in the first part but later chapters show that he is slowly opening up to him since Eden made Dawool realize that rather than being focused on the changes thats gonna happen in the future why not focus on the present. I think the fact that he still attended the alumni event, means he is trying to prove to himself that his ex doesn't faze him anymore but I guess his ex being a hypocritical ass, made him mad. Im not justifying his actions but rather pointing out the reasons what made him act like that.
The side couple's story isn't doing it for me. It's clear Dawool is still isn't over his ex and is using Eden as a momentary relief. I hate when a character is clearly not over their ex and they start a sexual relationship with the love interest before they develop a romantic attraction towards each other.