Martyr Without a Cause - A particularly dark example. Harry has been so conditioned to devote and sacrifice himself for his brother that even after breaking free from the web he still repeatedly chooses to sacrifice himself for others, even when this possibly isn't the only option.
More Than Mind Control: Lily drilled into him from toddlerhood that his only purpose was to protect his brother, in addition to actually, magically enforcing that conditioning..
Casting pain curses on himself to learn how resist pain
Losing on purpose to his brother for 'be as ordinary as possible'
The Unfavorite: An extreme example. Even aside from Lily's abuse of him in the name of making him a better servant to his brother, there are a lot of little moments where other adults, such as James, ignore or degrade Harry's achievements in favor of Connor. Many of them don't even seem aware that they're doing it.