This is very nicely done, with an interesting, persuasive take on 'infidelity' (and for the record, I don't really like infidelity stories, but I'm trying to approach the Games stories with an open mind and give them all at least a try, which I'm glad, because otherwise I might've skipped your story, which I quite enjoyed).
The slow progress of the relationship between Harry and Snape is well-handled, especially Snape's very gradual willingness to let Harry know about his life; he's such an insular man, with such very good reasons to be suspicious and untrusting, that it always warms me to see him getting past that. And his snarky, sometimes snarly, often very insightful comments are hysterical, as he continues with his long-established pattern of watching out for Harry, even when Harry is too clueless to realize it. Especially funny are the gentle digs at some of the well-established Snarry cliches--all the many ways Snape might've survived, the dingy, depressing rooms, the reference to his barely scraping by with semi-legal potions sales.
I'm glad this Harry is able--with Snape's help--to take a look at his life and actually *decide* what he wants to do with it, rather than just falling into what everyone expects of him, and I love the ending.