It's hard to remember, but Sam Vaknin (He actually is a diagnosed NPD himself, so that's an interesting take on it all) and Natalie Lue (although I spent more time reading her Baggage Reclaim posts, which aren't about narcissism specifically, but about boundaries/healthy relationships/how to break up and move on healthily etc)
I think Sam Vaknin was the one who said that a narcissist loves people in the same way a child loves a teddy bear. So, they genuinely do have a strong attachment to you, and they hold you and they are tender with you, but then something distracting happens, and they drop you on the floor and forget all about you for a day, a week, a month, whatever. When they come back, they expect to just start being tender again, because teddy bears don't get pissed off or feel neglected. This goes round the cycle until they get bored with their bear, and walk away from it for the final time. No goodbye, no closure for bear, no explanation, no empathy. There is just no further interest.
It helped me, that explanation, because I felt a fool for believing that the love was real, and it helped me to understand that it was - but just not in the same way that I love.
Pretty much anything that comes up if you search YouTube for Narcissistic Abuse/PD will help, because it's all validating, and narcissism really isn't complicated, so it'll be mostly relevant and accurate.