It is the first time or at least in her adult relationships. She's the one who left Charlie, she left Alex for obvious reasons, she left Pete because she'd met Oliver
Alex did dump her the first time they broke up. I remember because it later came out that he'd been cheating, and broke up with her after she discovered the cheating, and she still really clung to the relationship afterwards. She made that video after their first breakup talking about how she couldn't accept that she'd be better off without him, that she didn't agree with people in her life who were telling her she should be moving on.
It's hard to end a relationship when you've been cheated on, even though you realize it's what has to be done. I think it says a lot about Carrie's insecurity and inability to be single that she got cheated on
and dumped, and still clung to the guy. I think a lot of her cringy erratic behaviour now is caused by not having a new person to provide her with the validation she needs; she had her thing with Jack Howard during her breaks with Alex, Pete appeared pretty soon after Alex, we know Pete/Oliver overlapped in some way... but now she's just by herself, which she's clearly deeply uncomfortable with.
As far as I understand, according to some interpretations narcissistic behaviour fundamentally points to a pathological lack of self esteem, that makes people act grandiose to compensate. Carrie's latest post about her 'glad rags' really illustrates that she doesn't have it in her to feel good about herself on her own, she needs outside validation. She said it herself, when she doesn't have a boyfriend at home who can make her feel good about herself she falls into a pit. She doesn't have that quiet inner self esteem that would prevent her from needing to seek attention online with posts about how 'fierce' she is. I definitely understand needing outside validation when your self esteem is lacking, and that's why I also agree with everyone here recommending therapy. That's the kind of thing you can work on.