even showed this to my Italian girlfriend, her answer was something like "what the hell are you learning? Nobody would ever speak like that..."

I had the same issue... but just instead of will never have been try "won't have been"... i suppose it sounds better and it does allow it....

I was thinking the same as you because a couple of sentences before, we had: "Saremo ricordati per sempre" meaning, "We will be remembered forever". So, now we have a similar construction: "Non sarai rimasta mai sola" so I thought it would be: "You will never be left alone" but was marked wrong. Can anybody explain this, please?