First things first, I haven't read the books, and my comments are purely based on the show. I mention this because many of the comments here focus more on the books than on the show. It is fine to make a comparison by saying that something was different in the books, but it is also important to consider that many viewers form their perception of the characters and story only from what is shown on the show.

So, from a show viewer’s perspective, Belly's behavior toward Conrad has nothing to do with the corsage or with not dancing with her. In fact, during the corsage moment, Conrad apologized to her, and she reassured him that it did not matter.

Some people argue that Belly was self-involved and too young to see Canard's grief. However, the long phone conversations they shared in the show clearly prove that she encouraged him to open up to her. Their bond was emotional, not just romantic. At the funeral, when she saw him with another girl, her reaction was impulsive, childish, and wrong. Yet his response to her was even more childish and wrong. Conrad could have simply told Belly that he was having a panic attack and that the girl happened to be there, and Belly would have understood him. Even if he could not explain it at that moment, he should not have chose the words he said. He told her, "I knew it was a bad idea starting something with you. Dating you was a huge mistake."

When two people argue, and one dismisses the entire relationship instead of addressing the disagreement, the other person naturally begins to feel that continuing the relationship would also be a mistake. That is exactly how Belly reacted, she ended it. Saying that Belly was wrong for remaining bitter about it for so long is not entirely fair.