Draco might become withdrawn. He could isolate himself from his friends and spend more time alone in the Slytherin common room, brooding over what Harry has done to him.
One possible reason could be a misunderstanding. Maybe Harry misinterprets Draco's actions or words in a way that makes him do something that breaks Draco's heart. For example, if Draco is secretly trying to help Harry but Harry thinks he has some malicious intent and publicly shames him.
There's also 'Fractured Bonds'. In this fanfiction, Draco breaks Harry's heart by siding with the wrong crowd at a crucial moment. It could be that Draco is pressured into doing so, but the result is Harry feeling betrayed and his heartbroken. This often leads to a downward spiral for Harry as he tries to come to terms with what has happened.
Perhaps they are characters in a modern - day wizarding world where they are both part of a secret magic society. Edward could be a powerful wizard from an old family who feels threatened by Harry's new - found powers. So, he decides to break Harry's heart to assert his dominance.
Perhaps Tonks starts dating someone else, like Remus Lupin. Harry has some unspoken feelings for Tonks, and seeing her with Lupin makes him feel heartbroken. In the fanfiction, it could show Harry's internal struggle to accept the situation and his pain at seeing the woman he cares about with another man.
Another possible antagonist could be the Ministry of Magic. If they suspect Harry of something and force Tonks to spy on him or cut ties with him, it would break Harry's heart. The Ministry's strict rules and paranoia could be the driving force behind Tonks' actions that lead to Harry's heartbreak.
One possible plot could be that Snape reveals some long - held secret about Harry's parents in a very cruel way. For example, he might tell Harry that his father was not as heroic as he thought, and this shatters Harry's idealized view of his family, thus breaking his heart.
Well, it could be that Edward, perhaps jealous of Harry's popularity or a girl he likes, spreads false rumors about Harry. This causes Harry to be ostracized and his heart to break as he loses friends and respect.
At first, Draco might be in shock. He didn't expect Harry to break up with him at such a crucial time. Then he could try to reason with Harry, begging him to stay for the sake of their unborn child. When Harry refuses, Draco could turn to his own family for support and close himself off from the outside world for a while.