What if Harry knew
(1) he's a horcrux
(2) the full prophecy -- including the whole neither can survive while the other lives bit
(3) Dumbledores plan to let him be raised that way so he'd be willing to die for the wizarding world
And he knew it all at age 11?
And he knew it all because he was sorted into Slytherin and Snape has both a vow to protect Harry with his life and a duty to care for his snakes...
So Snape tells Harry basically everything. Where do they go from there?
Think about how Harry will feel. Is he betrayed? Is he still just grateful to be out of the Dursleys' house? Is he angry?
Think about Snape. He's lied to Voldemort, he can certainly keep Dumbledore from finding out that he told Harry the truth. But where does Snape go from here? Did he plan to tell Harry or was it spur of the moment? Does he have a plan for how to protect Harry from both sides of the war?
Think about Voldemort. Does Harry *really* have a reason to fight him? As of 1st year, Harry doesn't know a thing about his parents except what people can tell him about that night. Does it really hurt him so much that he wants to go fight some madman? If you still want Harry to fight Voldemort, develop it. Don't let it just be because everyone says he's a dark lord and he killed Harry's parents that Harry doesn't even remember. Have things actually happen over a period of time that actually makes sense for harry to want to go up against Voldemort.