Another bit of personal experience with NPD: de-intensifying thoughts.
You're probably going to have more luck with this in professional psychiatric setting because this entire process works better in guided and controlled environment, but it isn't necessary. Personally, I have a good amount of self-control and that helps. It might not work for you, just keep in mind there are other methods out there.
The idea is in diminishing intensity of emotions, thoughts, and conclusions once you notice that you're hit by a strong feeling or conviction:
"Everyone loves me" -> "There are people who love me and there are people who don't love me. There's also a great chunk of people who don't know me and it's illogical to expect them to like me without meeting/knowing me."
"Everyone hates me" -> "There are people who dislike me, but there are also people who cherish and love me. I am not worthless just because someone dislikes me."
"I failed at XYZ, which means I can't ever do anything right" -> "Failure is part of the process. The greatest minds of our generation had mistakes, bad days, poorly accepted art, etc. I am not talentless just because of one mistake."
"I haven't gotten supply in a while, I have to leave" -> "They might not know that I need attention as I didn't make it explicit. I can initiate spending time together or directly tell them I need to hear XYZ. I am not being fair."
"X disagrees with me, meaning their opinion is worthless" -> "They might disagree with me, but it doesn't mean they have no worth. If anything, someone willing to healthily debate with me is a person worthy of respect".
"Only MY opinion matters" -> "Actually, it will be really embarrassing if I end up being wrong. Very few opinions are worth dying on that hill. Keeping middle ground and establishing reasons why I lean towards X and not Y might be the best option".
"I have shown vulnerability, which means they see me as weak" -> "Most people do not consider vulnerabilty a weakness. In fact, me showing my true emotions most likely made them proud of me. I do not have to continue opening up if I'm uncomfortable, but it isn't a mistake that I did".
And so on. The hardest part is going to be to catch the emotion when it comes over. I don't have advice on how to become more mindful aside from actively paying attention to how you feel over the course of the day/week/month, etc. Maybe journal or create mind maps for your feelings. Whatever works for you, really.
This is loosely based around the general logic behind transference focused psychotherapy, which is something that can be used for treating NPD, as I recently learned. I am not a psychiatrist and my therapy is self-applied. With that being said, do your homework and don't trust me blindly.
P.S. Yet again, this is not universal advice so use your brain.