I've read this three times and am finally ready to comment.
This story is absolutely beautiful.
It's so rare to find an actual character study that includes a D/s storyline and you did this beautifully. I think too often most D/s fics are really about the humiliation, the kink and just finding ways to induce those things. This was the exact opposite of that. You wrote Snape so beautifully and gave his character so much room to grow from where he began in the story.
This was not just D/s for the sake of D/s. This was a story about a man not even realizing he should find himself, so much so that he's surprised when it happens.
Loved every bit of this and will revisit it without a doubt.