Thank you so much for your wonderful comments. I'm thrilled beyond words that you got so much out of it!
I've got a slightly more optimistic view of it, though. And Harry is the one looking back because he is the more demonstrative one, the one more open with his feelings, the one more willing to commit. His regret is for a relationship he has tried so hard to salvage, and Snape's is a longing for a relationship he either thinks he doesn't deserve or isn't fair to Harry. But I'm convinced that somehow, Harry will be successful in making Snape understand that they belong together.