Ohhhh, man oh man, your young Severus characterization here is brilliant. There are so many little details that just hit the nail on the head where he's concerned. Him being too proud to ask for charity in the opening bit. The way he refuses to get his hopes up or believe that what happened between Regulus and himself meant anything initially out of self preservation and the belief that no one could want him. Him being nasty and biting to Regulus when he resurfaces a year post-breakup out of self defense and anger. That he never lets go of his feelings for Regulus even after all the years in the "optional epilogue." Just so very him. Also, we can see him becoming the bitter, controlled man we know from the canon era, but he's not there yet; he's still got a lot of his youthful awkwardness and impulsiveness. Fantastic.
I love the way you've portrayed Regulus here as well. There's something Sirius-like in him; something about his mannerisms and arrogance and general air. But then he seems to very young and uncertain in the end when Kreacher's in danger. When he goes to Severus then the whole balance of power shifts.
The ending... oh, yes, perfectly painful. Finishing it with the letter... leaving Severus hanging in that way... wonderful. Although, gotta say, that epilogue is pretty damn fab too!! Guh, hotness, and LOVE Severus forcing promises and declarations out of Regulus in the heat of the moment. What a reunion!! And great last line. ::snicker::
The structure of this entire piece works very well also -- the present tense to past tense for the flashback and back again. Nice execution.