Same, I have Hyper Awareness and Social Anxiety ontop of a few other things... his reactions may seem extreme, but when you have body dismorphia and imposter syndrome it can be really difficult to cope. It's not that you don't trust the other person, it's that you're constantly bombarded with worst case situations that convince you you're worthless.
So, of course he would protect that part of him no matter what. The author even went to great lengths to show how accepting others were of him, but he couldn't see it. The moment someone found out, his ideal world started crumbling. But it was the best thing for him, because he got the acceptance he needed.
the story isn't perfect but i liked it, junseok's reaction to everything might seen dramatic but as someone with GAD i understand exactly how he's feeling and how his mind goes from 0 to 100 so fast.