Hi,
I don't have an opinion on what you should 'do' with regards to the relationship.
Regardless of what's to come in the future, it seems like you're finding this difficult, and I have empathy for your struggle and your girlfriend's struggle.
I do have plenty of experience that suggests that your girlfriend is far from alone in having these types of experiences. She won't be the first person to struggle to get grounded, helpful counseling or advice on these experiences, especially in modern day times when these experiences are seen as so crazy by so many. You guys are in a struggle, and it's a struggle others have had, but it's not a struggle that everyone recovers from.
For context, and to learn more about these phenomena, I can suggest a couple books and authors, both of whom are intellectually developed scholars, though obviously dealing with topics that are often marginal and marginalized.
'Supernatural: Meetings with the Ancient Teachers of Mankind' by Graham Hancock. (you can find Graham's recent TED talk somewhere on YouTube) This book contains dozens of accounts similar to your girlfriend's, from across cultures and time periods, including modern and medieval Europe. Might be good - if you are aiming to maintain the relationship - by seeing that these experiences are by no means a sign of pathological craziness or even events that are extraordinarily 'out there'. I mean, they're powerful and strange, but other people have had experiences remarkably similar to the ones you're describing, and these have been well documented. As a note of context, in some cultures, her experience would be celebrated as the beginning of a shamanic initiation, potentially leading her to be a powerful healer, hunter, wise woman, etc. I know that can sound ridiculous in a culture that routinely scoffs and huffs and puffs at experiences like hers, but that's kinda how it is, in a generalized sense.
Check out books and videos on sleep paralysis and other disturbing dream events, by Ryan Hurd. You can also check out his blog. He specializes in sleep paralysis and disturbing sleep events, without marginalizing them or calling them crazy. I know Ryan personally and believe he might respond to an email from you if you wanted direct advice or counsel, since he is probably the world's leading modern researcher on sleep paralysis, an event which often is accompanied by the presence of an intense spirit and can be incredibly frightening. These experiences can be worked with and changed!!
Since lucid dreaming may be a way that your girlfriend can get a bit more power in her situation, I would recommend Robert Waggoner's book 'Lucid Dreaming: Gateway to the Inner Self', to explore the many challenges and possibilities within the lucid dreaming experience. Hurd is also an expert and scholar on lucid dreaming.
Most of these book can be on your e-reader probably within minutes.
Big hug of support,
x