I join the chorus, I really liked the story. Your descriptive language is impressive. I more or less felt like Lily, watching television, since I had the feeling of looking at a picture or a film while reading some of the scenes. I loved the image of the angel they bring to life together and how they are shaken by its wing beats, a third presence between them neither is in control of. And in contrast the quiet tenderness of Snape folding his loved one in his arms, cradling their head, cherishing the moment.
I have only two very small critical remarks (or questions, really). First, I couldn't place the scene in the restaurant. When does it fit in the time line of Harry's and Snape's future? Or ist it another branch of that future? Second, I got confused about Lily using Severus or Snape when thinking of Snape. In the beginning, I got the feeling that Lily calls him Snape to keep her distance from him, and that it is an important turning point when she slips and calls him Severus. But in the end there seems to be no such distinction. Hmmm, I just realized that maybe this just shows that Severus and Snape have become one and the same person for Lily, which would probably fit nicely. So, nevermind.
So let me get to the end of my rambling comment by saying that the use of Lily as narrator was simply perfect. She allows for a unique understanding of Snape since she knew him in his formative years. And I guess this insight made so many moments of this story bittersweet.