....why is he stronger than I could ever be.... And not even exist?

Are you saying why hasn't he recovered from a traumatic experience with his first and main relationship, that carried him through the years where he developed his sexual identity and moved from the stage of being a teenager at school to an independent adult? Yes, because suffering a shitty abusive relationship during such formative years, and not having the resources to 'get over it" is something that can be resolved within the couple of years it has been since their break up. Jiwon presumably reinvented himself as "Alex" on line as a way to "get over it" in his own way, using distraction and avoidance. Now Dong-gyun is like exposure therapy for him, and obviously this is causing him distress.

(Also, not that it matters, but we don't really know when he split up with ex. He was shown with him at his school graduation. Do we know what age or year group Jiwon is even? He completed his 2 year military service, but returned to uni before DG so maybe he's older? The break up with the ex could have happened as little as a year ago, after his service, it might have happened before he left for the military)

Now that I've had a good look at what I had said and you're response, I'm kind of regretting it. ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄ I was kind of drunk at the time..... I'm sorry! And I am also trying to put together the pieces of his background. Jiwon is actually stronger than I could ever be. I could never get back up from such an experience.

No problem. I think it can be easy to just dismiss Jiwon as being a prick, because, from DG's perspective that's how his behaviour has came across (to speak plainly). However, when we look at the information given so far about his past relationship, and it's legacy, Jiwon's behaviour seems more of a self defense mechanism, even if it at times can appear to cause him and others distress.

Agreeable.