Hmm I think Jinwoo was more at fault, but obviously like you said, it's not only his fault. However, because the story is from JINWOO'S perspective, he blamed himself-- which is why the manhwa probably came across like that? I think the last 3 extra chapters are from Candy's perspective, so maybe we'll get more of what he thought there.
And also, I think in the end Jinwoo telling Candy to STAY, was showing how he wants to TRY again, not that he necessarily loves Candy just as he did in the past. You seem to be saying they shouldn't end up together because it looks like they don't love each other anymore, and only loved each other in the past. But that's the thing with old relationships and relationships that lose their spark through time..... like I don't think it's wrong for a relationship to take more EFFORT, and yes: clinging onto the past to keep it together, even if they dont seem to share a lot of common ground in the present. Because one day through that effort, they're gonna remember why they not "loved" but STILL LOVE each other. I mean that's a form of love too.
I appreciate you guys replying super calmly and I liked reading what you wrote, I just politely disagree. While the manga is from Jinwoo's perspective, the whole diary segment was not. That would've been a great moment to humanize the current Candy a lot but instead he became really flat (for my tastes). It's a DAIRY. Most people use diaries to vent and Candy was in a stressful situation yet he doesn't express any feeling other than helpless sadness for their relationship. His ACTIONS on the other hand, seem pretty frustrated, annoyed, perhaps even angry at Jinwoo. The saint that is painted in the dairy isn't really the same guy that coldly dumped Jinwoo and withdrew from his friends.
About the last paragraph, seeing how old relationships lose their spark and can be regained is something I love. I love seeing how complicated things get but that there's a silver lining at the end. However, I don't buy it here, I'm sorry. Jinwoo felt very nostalgic about their past relationship (which is completely understandble, especially in his circumstances) as did Candy, but there's really nothing keeping them together anymore. Which is what I meant with my last thoughts. What do they like about each other NOW? What unites them? They lead extremly different lives and have different goals so are they going back into a dying relationship out of nostalgia? I don't know, I guess I could clearly feel their love in all the flashbacks but I genuinely don't feel it in the present.
It's nice to hear other opinions, though!

How come the author decided to blame Jinwoo for everything in the end? They were both clearly to blame. They're not kids anymore, they dated for TEN YEARS and never learned how to use their words? That's just...I don't know. I'm sure there's people like that in real life, but I wouldn't consider that an ideal relationship. Could Jinwoo have put in more of an effort? Absolutely. Could Candy have voice any of his worries during the months before abruptly breaking up with him like nothing? Of course. And now that they're getting back together, I would've liked to see what they like about each other NOW. We know why and how they loved each other so much in the past but they don't seem to share anything other than an old love :/
Don't get me wrong, it's a cute manga. The art was beautiful they whole way through and it kept me pretty entertained. I just don't think that the couple ending together automatically equals a good ending.