This was very well done. Your characterization of both Severus and Regulus was wonderful. I think that this passage:
Regulus’ anger melted instantly, leaving behind an expression that caused Severus unimaginable distress. “Don’t do this; don’t reject me because you feel vulnerable. I know you’re scared, but – “
“I am not scared!” Severus shouted, disgusted at the look of pity etched on Regulus’ face, “nor do I need your compassion! Save it for those who are more deserving.”
Regulus shook his head sadly. “You mistake compassion for love, Severus, and there is no one more deserving of it than you. But if you’re content to let this drive a wedge between us, there is nothing I can do to prevent it.”
Was perhaps one of the finest Severus characterizations I have ever seen in a fan fic, hands down.