Well, people seems to think that learning to be a better person while having serious self-esteem problems are easy. It's not, believe me. Evety triumph seems hollow while every failure (and Jonwoo has got many) will take you into the pit again, even if objetively it doesn't.
Gaining self-esteem is the obvious solution but in that situation is difficult as hell, Jonwoo clearly don't do his part of it and the shit it comes to him doesn't help at all. Luckily Jon has good friends at his side and people he can trust even if he doesn't believe he can: Yuri, Hamin, Lila (trust me on this!), Taeyang (in his style, but I think he does) and, maybe down the line, Sungah. With then the definitely uphill battle it is it can be done, but it will take time and efforts an not just one breakthrough or moments of developement, but many until he believes he's the good person he truly is. Cut him some slack people, it will take long.
Regarding Sungah, I always though that she caught Jon's intentions with the confession, that's why she ended crying afterwards. This chapter confirmed me that, and maybe this can get interesting in time in that regard...