I do understand where Nietzche is coming from on the selfishness question. I guess a different word might help here, but for now let's use "good selfishness" and "bad selfishness". When most of us think of selfishness, we're thinking of the "bad" kind. A mother who neglects her children so she can play videogames, or who only buys enough food for herself is an example of the "bad selfishness". She hurts others without caring, and is ultimately self-destructive. "Good selfishness" is being able to meet one's own needs despite the objections of others, such as getting out of a dead end job (where your boss says they really need you) to go to college. Getting yourself out of an abusive relationship can also be an example of "good selfishness".
The struggle I always had with it was that I don't think altruism is necessarily a bad thing. I know that many cynics (including Nietzche) tend to give it a bad rap, but where would the world be if mothers did not put the needs of their children ahead of their own, at least to some degree? In that regard, altrusim seems to be more universal than selfishness, good or bad.
Take Nietzche with a grain of salt. Despite some brilliant insights (and he did have them) he was a pretty messed up guy personally, and didn't seem to be able to make his philosophy apply to his own life. My suggestion: take what feels right, and disregard the rest.