Was William really the golden child, though? I thin Charles and particularly Duana were very careful not to make out William was better. And William had to cope with his mother's paranoia that Harry was shielded from. He seems to have been a sounding board for advice that she often ignored. He was more aware than Harry of what was happening between their parents. It was him that forbade both his parents from coming to a school ceremony and invited Tiggy instead.And to add on top of that, he had no choices about his life. His future was ordained. I don't think William had some sort of blessed existence.
Although, to be fair, Harry might have very well seen it that way because he's so convinced that he always gets the worse deal in life. Instead of thinking about options of striking out on his own (with his father's financial support obviously), he just drifted along rather than actively planning his life. Diana, Charles, the Queen, they never treated him as the black sheep. (Harry says he was the the queen's favourite.) Maybe you could argue that the press did but then William wasn't falling out of nightclubs drunk, doing drugs (that we know about!), having naked pool parties. But none of this made him the black sheep with the public. It just seems like Harry never takes responsibility for anything.