Uncertainty
Despite Harry's hatred of Snape, up until a certain night Harry could grudgingly admit that he respected the Potions Master. Snape was on top of things, he was strong, clever, and didn't seem to miss a think. While everyone else hated Snape but accepted him merely on Dumbledore's word, Harry accepted him because of the small fragment of respect he had for the man.
This all changed the night Dumbledore was murdered. That's what it was, Harry had decided. He hated to hear people talking about Albus Dumbledore dying or passing away. But then they didn't know did they, they weren't there.
For days after the events of that night Harry wished for Hermione's logical reasoning to give a plausible explanation, he wished for McGonagall to snap and him for his hatred of Snape. Most of all he wished to hear that quiet firm voice, to see those piercing eyes and feel safe once more.
Instead what he heard was moans and self-hate as everyone vocalized their regret in not believing Harry. Harry liked being right, he just wished he had been right about something different. It was hardly a triumph, when you witnessed who was the greatest wizard alive be murdered.
Worse yet, in Harry's mind it was no longer a mission to rid the world of Voldemort, it was a mission of vengeance against Severus Snape. A small part of him knew his full concern should be the one who had given him his scar, but he knew he couldn't ignore what Snape had done.
For the first time Harry was uncertain of the future. Uncertain because the one who had always guided him wasn't here to guide him anymore, that in itself was a difficult thing to comprehend.