How do you know what was true and what wasn’t in a narcissistic relationship?

When narcissistic abuse survivors look back, they remember the good times that they shared with their partner and get bogged down in confusion. Did they get it all wrong? Surely, the good times were the way they remember them? What are they to make of the whole relationship? Could the relationship be worth saving? And are they the party at fault? What really happened in the relationship?