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.
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.
The antagonist could be a cursed object. This object, which has some dark magic in it, might be found by Harry. And when he interacts with it, it starts to control his actions, leading to him breaking Ron's leg. So, in this case, the cursed object is the source of the trouble and can be considered as the antagonist in the fanfiction.
Well, in this kind of fanfiction, Dumbledore himself would be the main antagonist since the title implies that he is the one 'breaking' Harry. He would be acting against Harry's well - being and goals.
Another possibility is that there is no real 'antagonist' in the traditional sense. Instead, it could be the circumstances that act as the 'antagonist'. For example, the differences in their worlds, their duties, and the fact that they come from such different backgrounds could be what causes Erza to break Naruto's heart.
A jealous wizard or witch from the Order of the Phoenix could be the antagonist. They might be envious of Tonks' relationship with Harry or her special abilities. So, when the potion mishap occurs, they try to make the situation worse for her, hoping to get her removed from the Order or at least lose her status among the members.
Maybe it's a jealous rival of Naruto who wants Hinata for himself. He manipulates situations in such a way that Hinata ends up hurting Naruto. For example, he could stage events to make Hinata think Naruto has betrayed her trust.
Ino could be seen as the antagonist in this fanfiction. Since she's the one breaking Naruto's heart, she's causing the main conflict. Her actions are directly opposed to Naruto's happiness, which is a typical trait of an antagonist.
The pure - blood family that arranged the marriage contract could be the main antagonist. They might have a lot to lose in terms of status, power, or wealth if the contract is broken. So, they would do everything in their power to force Harry to uphold it, even using dark magic or underhanded tactics.
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.