Occlumanzia
A spell that Harry certainly did not master during his time at Hogwarts was Occlumency, the obscure and ancient art of closing your mind against Legilimens, those who intrude upon other people’s minds. In terms of difficulty, it is probably on the same level as the Patronus Charm, but both demand very different disciplines, one which Harry excelled at, and one he just couldn’t nail.
Because while the Patronus Charm requires you to embrace emotion, Occlumency requires you to empty yourself of it – to clear your mind. Only then will you be able to prevent a Legilimens from discovering any feeling or thought, because there are no feelings or thoughts to discover.
Draco Malfoy attempts to attack Dumbledore under Lord Voldemort's instruction.
So Occlumency requires a certain type of mentality to work. This is why Draco Malfoy, who had to use Occlumency to hide his plan to kill Dumbledore in Harry Potter and the Half-Blood Prince, excelled where Harry had failed.
In an interview in 2005, J.K. Rowling even confirmed as much: ‘I think Draco would be very gifted in Occlumency,' she said, ‘unlike Harry. Harry’s problem with it was always that his emotions were too near the surface and that he is in some ways too damaged. But he’s also very in touch with his feelings about what's happened to him. He's not repressed, he's quite honest about facing them, and he couldn't suppress them, he couldn't suppress these memories. But I thought of Draco as someone who is very capable of compartmentalising his life and his emotions, and always has done.’
