Hey hon, I'm sorry that this is happening to you. We can't diagnose, but I can be a good guesser!

You said something like she's in constant fear of a break up- sounds like fear of abandonment.

Also that she switches positions instantly....

I would say her symptoms seem to lean towards Borderline Personality Disorder (BPD) by just what you mentioned. Of course she'll need to be properly diagnosed and treated by a qualified mental health care professional.

This can be very difficult to live with if left untreated. It requires extensive therapy sessions, DBT, and possibly some medication. The medications are similar to the ones that are used in bipolar disorders, like the ones listed below in my signature.

It'll never get better. She can like you one minute then despise you the next. Like you said, it's not a thought change, but a personality change.

Early childhood trauma/abuse can cause such personality issues. It can develop to a level of secondary structural dissociation, much like Other Specified Dissociative Disorder (OSDD)- not to start a scare or anything, it's just the order of progression.

If there is some way that you can get her help, you must. What you see now is what you got. To her, she's always been this way so it's her normal which means she'll be in denial of anything being wrong. Things won't get better until she gets help if it is BPD, or any other disorder actually.

I know what switching between personalities is like. I never noticed before in denial, but I know now and is aware of it. It was my normal for all my life, so I wasn't bothered by it.

I wish y'all luck.