Oh. My. God. I just absolutely love this. This is the Severus that lives in my head - sad and scared and guilt-ridden, but detached, determined not to make himself vulnerable or hurt anyone else he cares about.
I'm a bit in awe of the sex scenes. The way you wrote it is somehow deep, intimate, visceral, and hot without actually being that explicit. I felt like I was feeling what Severus must be feeling - confused, uncertain exactly how it had happened, unable to understand why, but equally unable to stop because it tugged at his heart and he needed it. I am, admittedly, a little confused about how you had managed to do that, but I sure liked it!