I've been through the same. Hang in there! Things get better with time.
You're correct that it makes sense in his current state, but his behaviour is still that of a brat and selfish, the difference is the reasoning. And that's coming from someone with severe mental health issues who also relates to him, but I can also admit that the behaviour my mental health causes me is that of a brat, because your mindset changes. So both you and the people calling his behaviour ridiculous are correct, but having mental illnesses doesn't mean their bratty behaviour suddenly isn't bratty, it still is, but just has different reasoning ╮(￣▽￣)╭
Some people are blaming Junwoo and saying things like 'he is a brat or spoiled". Those people obviously don't know anything about mental health. Every word people say to you or every advice they give to you seem and sound like pity and not genuine because you're lost in your own negative thoughts. You think you're nothing and you don't deserve anything good to be happen to you. You think nothing will get better even if you try. I'm also somehow in this phase right now and it feels like hell. Believe me. Sometimes I have these negative thoughts too. I don't want to die. I just don't want to live. I just wished I never existed in the first place. So I really feel a connection with this manga.