This one is blunt, but bear with me. Sometimes the only way to stop the cycle is to rip off the band-aid quickly.
The end of every relationship with a narcissist ends with a discard phase. This phase is terribly confusing and painful, and they show no mercy. Even if you think revenge is a good idea, it never is. Their ego will not let you get closure or a last word.
Once the narcissist has found you to be weak, they will disappear. Their true selves are showing even more. They no longer value the attention from you they once demanded. It just is not enough; nothing you can say or do is ever enough. Yet, they may project that on to you by saying you never do enough. This is very confusing for the victim, who still feels as if this person is the love of their life.
The eggshells you have been walking on are now everywhere. There is no safe place to step, no question is safe, nothing you can say is right. The act they put on to get your attention became too much for them, and now they no longer even try. You have no value in their eyes.
The narcissist will withdraw at their will, sometimes coming back to phase one and doing it all over again, other times completely. But they will usually come back again and again. And in doing so will destroy the victim's self-esteem and make them question if they were ever enough. The worst part is there will never be an apology. The only way to really stop this is to have no contact.
The most helpful advice is to be prepared. Know that when you are replaced, as you will be, that the narcissist has been looking to replace you with a new energy source, and when that one is used up, that is when they return. At this stage, be strong. Shut the door, block them on your phone, fill time doing things you enjoy with friends and just ignore them… Time will pass. They’ll disappear (eventually). And you will feel better again.