My N-mom has never managed to keep friends for any longer than a few years, with one exception: a woman who is extremely dependent on my mother for emotional support. They're like a textbook example of codependency.
I do know another N pretty well; my sister-in-law. She is always,
always talking about her friends, saying "my friend so-and-so did such-and-such", on the phone to them constantly, and most annoyingly, she judges others on how many friends (she thinks) they have. Several years ago, when I'd only known her for a year or so (my husband and his entire family had warned me about her, to their credit), all of a sudden one day she went off on me about how I was a failure in life because I didn't make new friends easily. When in fact, I've simply never talked to her about my friends.
She makes "friends" very easily and uses their achievements and actions to
her credit. It's hard to describe how, but she manages to brag about herself through her friends! Lately she's been going around the world: "I'm going to to [major city] to see my friend. Somehow I just make friends in such great places!"

When she gets back she'll talk of nothing but her "friend" and how perfect they are. Yet she's never invited by the same friend twice.
People who don't know her very well think she's a fun-loving international socialite; people who truly know her
stay away because they know better. Those who are currently her friends think she's the greatest and simply can't imagine how anyone could criticize her. It's almost comic when these friends visit her family, because her whole family knows she's an N, but they've turned dealing with her into an art form: ignoring her narcissistic nonsense and only engaging in neutral situations. So when she brings over friends, my family-in-law's normally relaxed, talkative atmosphere (what they're like when she's not around) turns into blasé silence.
I understand what you mean by not getting it... I mean, how can they find so many people to feed off of? How can people not see through it? Some seem to, but we don't know for certain since they're attached to her and no one else. She makes certain of that, and always blames separation on the others. I'm the first person unrelated to their family who's ever gotten to know her well and also stayed close to them. She started off by being sunny and carefree, very helpful, then began telling me how her parents couldn't stand having me around but didn't have the courage to tell me to my face, so she was taking a stand for them. "It would be best if you just left," she said one day, with a very serious, concerned look on her face.
She didn't count on me also seeing the hatred and hypocrisy in her voice and look, so familiar to me after being raised by an N mother. I approached her parents and politely asked if what she had told me was the truth, knowing it wasn't because I'd experienced acceptance and joy with them. Indeed, my SIL had lied through her teeth. So now I understand how it works: when Ns get tired of their plaything, they poison the well and have no qualms whatsoever about lying to get rid of their ex-friend as thoroughly as possible. It's as simple and disgustingly heartless as that.
That's great that you've found relief

The "is it my fault?" feelings will stick around for a long time, but you've already learned the most important part: to listen to your inner self saying "NO!" Very well put!