I've been pondering my comment. I'm not big into infidelity fics, been hurt to much that way, so this probably colors my impression.
This was well done though. True to Snape, the infidelity was subtle and Harry was never the wiser. This was probably good for Harry since we don't know why Snape was cheating. Snape seemed to care in his own way for Harry. And he seemed to be trying to help Harry in all ways that someone who loved another person would.
I am very happy that Severus chose to end the affair. I hope he doesn't take up with anyone but Harry from now on.