As for blaming your mom, how long do you think you can do that? Until you’re 30? Until you’re 40? Until you’re 60? Doesn’t it seem logical that at some point you need to take responsibility for your own actions and behaviours? It sucks that she didn’t give you all the tools you need to be a good member of society, but very few parents actually do - we all have to figure out what we didn’t get from our parents and get it for ourselves. The good news is that that is completely empowering - it’s within YOUR power to get what she didn’t give you, instead of blaming her for it.
Something that works/worked for me, when this “demon” wreaks havoc (counting to 10 did nothing for me either) was, as soon as it starts happening, yell “STOP!!!” (not verbally yell, but make it a loud, sharp internal directive). When I did this, I imagined a machine (“demon”) skidding to a halt in my head. Over time, the “demon” has bothered me less often, and with less intensity. It helped clear my mind so I could figure out what I was doing wrong, and how to fix those things. Good luck.
You can re-parent yourself in some ways, with effort.
When you’re in the shower every morning, loudly and with intensity, say, “Today I give myself permission to not be a narcissist!” Yeah, I’m aware of how silly it sounds, but I’m the proof it works. And I was raised by the same kind of mother as you, I know it ain’t easy.
The one warning I would give you is, when you are still young and rubbery is the only time you get take an active role in shaping who you are. If you wait too much longer the mold will be cast and changing such things will grow more difficult with each passing year. And you can’t change what you don’t own, it’s time to stop blaming Mom.
Cognitive behavioral therapy. Cognitive behavioral therapy. Cognitive behavioral therapy.
Don’t spend too much time trying to figure out why you’re a narcissist. I’ve seen fantastic parents end up with a narcissistic kid, I’ve seen horrible parents produce amazingly well-balanced kids. While nurture plays a role, lots of times one’s deck is heavily stacked by the genes they were dealt. So work on the cure, not the cause.
Oh, and get into cognitive behavioral therapy. AKA CBT as mentioned in many posts above.