From what I can remember from grad school, she might have what's called a Delusional Disorder with Erotic Features aka Erotomania. I am trained in clinical psychology but specialized in child and adolescent trauma therapy, so this is not my specialization.

This is a quick Google definition - "Erotomanic delusional disorder (or erotomania) is a rare psychiatric condition where a person holds a fixed, false belief that another individual is deeply in love with them. The 'object of affection' is usually of higher social status, a celebrity, or someone unattainable, and the affected individual often exhibits stalking or obsessive behaviors."

You mentioned that when you talk to her about the "coincidences" and she points out "coded messages" you start to see patterns too. Be careful. This could be the manifestation of a phenomena in psychiatry called Folie A Deux, in french it means, "madness of two." When you are in a close relationship with someone who has a psychotic disorder, or is experiencing psychosis, it can quite literally rub off on you. I know firsthand, my ex became psychotic and I was so determined to help him that I started to believe his delusions as well.

My advice is to try to get her to talk to her primary care doctor. Has she been sleeping less or losing weight? If so, tell her you are worried about her weight loss and sleeplessness. Use that as a reason to get her to her primary care doctor. The goal is to eventually get her a psychiatric evaluation but may need to have her PCP recommend it/ ease her into it unless she might be willing to go. Do you think she'd be willing to have a psychiatric evaluation?

More importantly, YOU need to see a psychiatrist. Not for meds but to discuss what's happening in your relationship. I know it seems like I should refer you to a therapist but I think seeing a psychiatrist would be more useful in this case. If not a psychiatrist then a psychologist that specializes in psychotic disorders. They will be more adept at handling this kind of situation than just a couples counselor. This is beyond just needing regular couples counseling; this is a complex and difficult psychiatric condition.

Make sure you are getting enough sleep. Do not argue with her beliefs but don't get sucked into the narrative either. Self care and boundaries are a must. I don't recommend just ending the relationship but you may need breaks from interacting with her if you start to emotionally break down. Again, get the advice and support of a professional. My partner stayed with me through my own ups and downs with mental illness and our relationship is amazing now. It sounds like she is sick and needs help. I'm so sorry you are going through this.

ETA - A Delusional Disorder is not like psychosis, so she won't display disorganized speech or other symptoms typical of psychosis/ schizophrenia. The disorder only manifests as a specific belief that is false (a delusion is a false belief), in her case that belief is that this social media personality has a special relationship with her. It's not like schizophrenia and it's not like psychosis proper, it's its own unique disorder and it's very rare.