I'm trying to find a fic which I think was written for a fest, but I'm not sure.
Harry and Severus are in an established relationship. Severus somehow messes up an experiment and he realizes that he is dying.
[Spoiler] Out of guilt, for having to leave Harry alone, when he dies, brings another Severus to their universe. But the other Severus doesn't like Harry and they had never been together. Harry assumes that he just doesn't remember, I think. Slowly, he realizes, that this is not his Severus and he starts to hate him and treats him badly while I think Severus falls in love with him. Harry is very angry and bitter, because his Severus acted so selfishly.