
I totally get that Eunho is very deeply hurt, and the way Jaeha broke up with him was very... abrupt? Cruel? Whatever you want to call it. However, you are only taking into consideration Eunho’s perspective.
Jaeha is the one who suffers the most. He truly believes he has no value as a person, constantly putting himself down. Did you actually read the flashback chapters between season 1 and 2? Did you see the way he was treated at home? The way that teacher took advantage of him and then threw him aside like he meant nothing? Then he finally meets this girl, Sun, and just when he thinks he’s happy, everything gets messed up. I mean, in Jaeha’s mind, he can’t ever be happy, and he believes that just like with his marriage, everything will go wrong in the and and he’ll just get very badly hurt (and those around him). I do believe that he also thinks he can’t make Eunho happy so he rather not be by his side, but the truth is he’s scared. Can you blame him?
I do understand them both, but I believe Eunho is being too inmature and too selfish. Sun did understand Jaeha, she realized the problem was within Jaeha and the fact that he didn’t rely on anyone, but she thought there was nothing she could do. Eunho’s just forcing Jaeha to change, but in a way that’s transforming their relationship into a toxic one. If he was more mature and understanding, he would realize that he needs to stay by his side, to be there when not even Jaeha is there (if that makes any sense), to show him he can rely on him and that he won’t leave him. Because love shouldn’t be about yourself, it should be about the other person. Even if Eunho means well, he’s hurting Jaeha. If he can’t handle it then he should just leave and let him be.

I actually think Eunho is pushing Jaeha to deal with his internalized insecurity so that he can open his heart and let someone else in. They have both hurt others some way or another (Eunho by creating the toxic relationship that they are in; and Jaeha by not being brave enough to own up to his true feelings) But they did those things due to other factors that we can agree are understandable. Just like you said, Jaeha has experienced events that made him pessimistic and self-loathing. And like the person before you stated, Eunho has been deeply in love with Jaeha for 10 years before getting rejected by Jaeha for unclear reasons. If Jaeha was a lot stronger mentally, maybe he would have been able to overcome the things people did to him with a more positive mentality. And if Eunho was more mature, patient, and understanding, maybe he would have dealt with Jaeha's rejection differently. But I also think that because of Eunho' youthful impatient and pushiness, he was able to make a crack at Jaeha's shell and convince him that Eunho trully love and cherish him. I feel like, or at least hope that, their personalities actually compliment each other and can help each other to find happiness.

I totally agree with you in that they both have done things for understandable reasons. Like I said, I do understand Eunho. But I've seen that not many people try and understand Jaeha, thus my comment. However, I still think there is a difference between the way they both act.
Even though I agree that if Jaeha was less insecure and braver the situation wouldn't be as bad as it is (which means he's also at fault), I think that this condition is a consequence of very awful experiences he has had throughout his life. On the contrary, Eunho's behavior is due to him not being mature enough, even though he believes he's doing it for Jaeha. And that becomes clear when he said "I need to understand why he keeps running away". At least I got that impression, that he's thinking of himself first, instead of respecting Jaeha's pace. I do agree with you that with his pushiness he has managed to get Jaeha moving in the right direction, I also believe that was necessary. But with his attitude now, adding manipulation, domination and guilt into their relationship I just cannot accept it. I totally get that being in love with the same person for 10+ years is hell of a long time, and having all your dreams shattered like that must be terrible, I know. BUT, that doesn't lessen the fact that he isn't looking beyond the superficial problem and that he has no right to blame Jaeha. And if we're getting into that, Jaeha has also had his dreams shattered not one, but multiple times. Of course I'm pretty sure that the story will end with Eunho being able to get Jaeha to overcome his insecurities and all, but I just don't like the overall message of it. That's very subjective though xD
(Sorry I get carried away, but I enjoy debating like this (▰˘◡˘▰) )

I enjoy debating too:) Although I must confess, I have skipped/skimmed through some chapters so I can't really have a wholesome view on the story. I just really love the kind of story where two human beings with their flaws and everything fall in love and make each other better. It is like the perfect mixture of reality and fantasy. So I am hoping that the story will end like that. Also, I think I get your point. Jaeha did everything he did out of fear which was created due to his past experiences, while Eunho seemed to be doing these things in order to get back at Jaeha (even though he, himself, thinks he is doing these things in order to get close to Jaeha by being a sex friend rather than demanding more intimacy, thus making Jaeha more comfortable with him.) I don't know what Eunho's true maybe subconscious thoughts are about his manipulation and guilt-tripping. He should be feeling pretty hurt that Jaeha rejected him. Also, don't forget that Jaeha is supposed to be the adult in the relationship but he is acting quite like a coward. But I feel like that's a common theme in story revolving around bigger age gap couple (such as How to get a sugar daddy and other couples that I can't remember so I can't be very sure about my following theory). I think it got something to do with the boldness and fearlessness of youth and the more experienced cautious adult. So I still feel like they have both hurt each other pretty bad. Although, Jaeha has done so subconsciously due to his reservation to relationships; while Eunho is doing so maybe out of desperation and a bit resentment.
That boy loved him for over 10 years ... he saw him get married ... saw him with that friend ... finally got a confession from his hyung ... they were lovey-dovey and happy ... and then they make a trip together ... everything is as eunho wished and then .... and then he breaks up with this boy without a proper explanation
Seriously after seeing what this boy went through ... how do you expect him to behave?
I don't like jaeha that much, but since eunho loves him i won't mind them ending up together