What a lovely story! I like the way you've described each place they visited, and how your snape is sensitive and uncertain of himself, but not so much so that he's out of character. And harry, well, they're both spot on for how I picture snape and harry.
This line - "I can't stand it when you're nice to me then take it away."
killed me. It's so simple, and perfect for harry to say to snape.