He could also develop a sense of determination to prove himself. Instead of wallowing in self - pity, Harry might strive to show his parents that he is just as worthy of their love and attention. He could study magic harder, practice his skills secretly, and look for opportunities to shine on his own terms.
Another reaction could be anger. Harry might be angry at his parents for their neglect. This anger could manifest in small acts of rebellion, like not following their rules or speaking out against their unfair treatment. He might also start to distance himself from the family unit as a whole, feeling that he doesn't belong.
Harry might become withdrawn and introverted. Feeling unloved and unwanted, he could start to keep to himself more, spending time alone in his room or exploring the grounds of their home alone, trying to make sense of why his parents seem to favor the twins.