Hi Fooled.
I feel your pain.
So much of your story resonates with my experience with my BPD ex girlfriend.
Normally I would now write a long post echoing what the others here have just written (and I agree 100% with what they've said).
But I'm going to keep this post short, because I think it's important that the point I'm going to make isn't hidden or diluted with excess text.
You want to believe that they are the nice person you met at the start, and that the bad behaviour and disrespect is them "acting out" because of something that happened to them.
The truth is that the person you met at the start is a facade, created to charm you, and the person you saw emerge as time went on is the
real them.
And whether they're BPD or NPD, they cannot be fixed.
Your co-dependency is most likely telling you that you can be the one that will fix them, protect them, help them change, because they think more of you than they did all the others.
Truth is, they don't think any more or less of you than any of the others.
If they're BPD, then you're just a life raft that they'll hold onto while they think you'll keep them from going under.
If they're NPD, then you're just a means to an end. Someone they can play games with, cheat on, lie to, and string you along until you're no longer needed, and then they'll jump straight to someone else without blinking.
It is painful, but it's important that you come to terms with it.
If you go back to her, it will be a lot worse, and it gets worse every single time you let them back in because they have to punish you for leaving them (even if they left you).
Read many of the stories on here for the living proof.
If you're struggling believing she could really be BPD/NPD, ask yourself this:
Could you treat her the way she treated you?
I'm guessing you couldn't.
Relationships are meant to be balanced.
It's clear, regardless of her mental illness, that you're not meant to be together.
Being with her would be very damaging to your mental health.
If you still struggle, try to remember it this way.
Their emotional development was arrested at a very young age, and they will be stuck there for the rest of their life, whether it's as a 4 or 5 year old mentally, or a stroppy teenager mentally.
Just like kids of that age, they will act immaturely, tantrum if they don't get their way, show no remorse, be very selfish, get bored of people very quickly, lack social skills and respect for others, and expect not to be held accountable for any of their behaviour.
And if you do call them on any of their ridiculous behaviour, no matter how small, they will have a hissy fit.
Now, if you saw that behaviour in someone of a young age, you'd have no problem in shrugging it off, maybe even chuckling because it's how kids act, even if they were directing that behaviour at you.
But, because it's an adult with the emotional intellect of a child, it's harder to comprehend.
We convince ourselves that they are normal, rational people.
They never were.
They just hid it well for a while.
I know that right now, the pain seems unbearable.
We've all been there.
It does get better.
You just need time.
This is a longer post than I meant to write.

I'll leave you with a book I recommend to people very often.
It helped me a lot.
Psychopath Free by Jackson Mackenzie
I know the title sounds harsh, but trust me.
You'll recognise a lot of what's written in there, and it will help.
For more immediate help, check out these youtube channels.
The Little Shaman:
https://www.youtube.com/channel/UCnr7eQQzbj01-Js_Exsr6vgJess Stanley:
https://www.youtube.com/channel/UCm_dWUsXjZrg9aXmUKexnwgBoth these women have been in Narcissistic relationships, and are very knowledgeable on the subject of Cluster B mental illnesses.
And try and remember all the BAD things that she did to you.
The brain will try and forget them and will try to only remember the good times, giving you a rose-tinted view of your relationship.
That's normal, but you can't let it happen.
You know you were in an abusive relationship, but you're just having withdrawal symptoms right now.
Fight it!
Hang in there mate.
Arm yourself with knowledge, binge-watch those YouTube channels, and remember that it's not your fault.