WOW, this was absolutely blow-my-socks-away fabulous! Such beautiful language, and such a perfect resolution at the ending. It's odd to say this about an infidelity fic, but I can't think of anything that's more romantic at it's heart than this:
It was the ballast of his days, Severus knew as he turned down the sheets. Harry climbed in and began talking again, filling the dark with half-sensible murmurs of extraordinary brooms. At length he fell quiet; he was dreaming of blue pine. Severus caressed the motionless fingers that had wrapped about his wrist, his strokes slowing as he dozed, his thoughts drifting to his potion and white briony floating over a three-quarter flame. I have all I need, he'd told the shopkeeper, bowing away from the gentle tug at his cuff.
I know that a lot of people don't like infidelity, but to me this is perfectly realistic, and I think that the characterization is perfect here. You've really captured them both - I love the way that Harry natters on about brooms and Severus has his potions and they still just fit together. They're perfect.
This is a beautiful, beautiful story and I'm still flailing and breathless at the ending. I can't remember when I last read an ending as perfect as that one ... it's just ... wow.