Alright, I found everyone's reaction over the years very interesting. So if you read the manga when you are not in the haze of lust, it is quite clear that the story is quite basic and not that great as well. However, this doesn't mean it isn't relatable especially if you had experiences with unrequited love. One thing to come to terms with is Chiaki is indeed a bad person, maybe when both were smaller and didn't really know better this was understandable but when they "grew up" it is not acceptable. At that point all she can do in good faith is either talk things over with the guy or completely remove herself from his life. Her coming back and onto the guy is disrespectful to everyone included, not to mention to her bf which is being cheated on. A better story would be if they met each other again in highschool where they are at the height of their adolesence, or if we are to keep the core timeline intact making the epilogue of how their adult life turned out in terms of relationships, did the guy understood he was emotionally used by a narcissist? The Chiaki ever realised she was horrible to both guys? Was the girlfriend alright with being someone that was hung up on another person? The story is cut prematurely and not in the most clever way, it uses this sense of bittersweet relatibility to mask the fact that narrative didn't go forward much.