Most of us see the unmasked narcissist only during the discard phase. This is when she decides you no longer meet her needs, and she cuts you out of her life, with about as much regard as most people give to shooing away a fly. You've become burdensome and bothersome, and that's why she see's now sending you away. Up until this point, she's been your "soul mate." You've shared good times and you thought she was your best friend. However, it was a one-sided relationship, because she was using you, either for narcissistic supply or for a sympathetic ear because she is always in a state of crisis. You were her sounding board.
Or, perhaps she wanted you to watch her children so she could have some time to herself, unencumbered by the constraints of raising a family. (Narcissistic mothers aren't really too invested in their children, despite their protestations to the contrary.)
For whatever reason, she no longer needs you, or wants you around. Maybe she realizes you've caught her without her mask, and the glowing first impression has worn off. Because narcissists need constant praise, she's now getting it elsewhere.
However, if you think about it, even before she decided to discard you, and move on to the next victim, there were subtle warning signs. Occasionally, a malignant narcissist will slip and let you know what she's really like, and how her mind really works, even during the idealization and grooming phases, where they "love bomb" and flatter you.
Here is an example. One morally disordered person whom was once a big part of my life let her true colors show, when she suddenly dumped someone else and said, "I don't need them anymore," or something to that effect. She also dropped another "friend" over a very trivial matter. These are red flags to watch for, because she the person in question might turn out to be a fake friend.