Then, Harry went to a new optometrist, and discovered color contacts. He tried a pair in blue: Snape looked in his eyes that looked nothing like Lily's, realized he loved Harry more than life...the end. *g*
Okay, so my ending doesn't fit at all with the tenor of your story. The ending you used, in fact, was brilliant. Not every love is doomed to succeed, and not every past can be overcome. That is why JK felt she needed to kill Snape, I believe: she never thought he could atone for his sins. Maybe she's right.
Certainly, it doesn't seem as if he will learn this lesson, and he is not worthy of Harry quite yet. He doesn't yet understand his own advice to Harry, which makes it doubly sad. He cannot live for another; he needs to learn to live for himself, and begin to enrich his life, rather than continue to pauper himself trying to factor its costs.
This was wonderfully written, with a great flow, and a very natural, instinctive storytelling. As always, your description makes the setting come alive. Thanks for an excellent foray into relationships, especially because you took the less traveled road.