It sounds like the truth is that she didn’t meet your relationship needs and that it took awhile to figure that out, probably because whatever dysfunction you shared — the reason you were drawn to her in the first place — made things both euphoric and awful.
The euphoria was addictive and the awfulness was your needs not getting met.
That’s what “it’s not you, it’s me” means.
It means, “You’re not a person who meets my relationship needs”.
And that’s okay.
It’s healthy to be able to walk away from the euphoria and say, “This doesn’t work for me”.
It doesn’t mean you are broken or damaged.
It simply means you want to find people who do meet those needs.