Answer:
Harry is afraid that he is becoming more and more like Voldemort because of the strong connection which exists between them. Harry talks to Sirius and confesses to him that when Mr. Weasley was attacked, he was not just watching the attack. He also says he was the snake and reveals to him that he felt the same thing while he was standing at Dumbledore's office.
Harry is afraid of his future. He considers he will eventually become like Voldemort because he feels anger all the time and thinks he is becoming bad. Harry says that the past events and the similarity he shares with Voldemort may have a huge impact inside him, by converting him in a bad person.
Harry and Voldemort both grew without parents and they were unloved. However, Voldemort chose to become evil, while Harry chose to be good. Through his mental connection with Voldemort, Harry is able to see some really bad scenes from his and Voldemort's past (e.g. when Voldemort killed his parents, how he ended up with Neville's parents, etc) and that makes him angry and full of negative thoughts. Because of that, Harry considers that his connection with Voldemort and his ugly past will change him for the worst, although he is putting his best efforts to be a good person.
However, Voldemort never managed to defeat Harry, by converting him into a bad person, as Harry has something that Voldemort never did - love.
