I've seen a situation similar to this happen around me. Imagine it as your situation in an alternate universe if you will. My room mate, she's jewish, but she doesn't practice judaism, she ended up marrying some really cool german dude, and their all fine and dandy.
Your arising jewish woman problems are awkward as hell from the way you wrote them. She's in love with you, but she can't be with you because you're not jewish...so you're the forbidden fruit she delights in too much?
And, as most of you probably know already, it is NOT ok for a Jew to be in a serious relationship with a non-Jew
No, most of us don't know. How about not following this idiotic social construct either you or others have put in your head. Plenty of jewish people marry non jewish, and many jewish people themselves are not 100% jewish either.
and she, being a good Jew, believes in this tenet (to what extent I'm not really sure), regardless of her feelings for me. This all finally came to a head tonight in a conversation that was somewhat surprisingly muted and unheated, with both of us deciding to stop the discussion tonight after agreeing that it's a serious problem. I must note that I am more foolishly/hopefully optimistic than she is at the moment.
So, she put this belief in her head regardless of whether or not she believes in the religion or not. I would remain optimistic for the mean time. It sounds like there is plenty good that will keep you two together. If it were me, I would be trying my damndest to keep her from breaking up the relationship because of some ancient and archaic tenet that people don't follow any more. This tenet that jews should only be with jews is old and archaic as it arose from a much different time long long ago, and the time it arose and was designed for has much since passed (this tenet is outdated, no longer applicable, and has no place in modern times).
Do your hardest and don't give up on her. However yet again if it were me and she did break up because of the outdated tenet...I would be so pissed and never see her again (not even remain friends). It'd be extremely insulting to me to be tossed to the side of the road because of something like this.
You're not me which is fine. I'm just painting out what would probably happen for me if I were in the same situation since it is a consolation for you looking at this situation from different perspectives currently. Definitely ask her why she follows this tenet, it makes no sense for her since she already violates it, and there's not much sense in her following it anyway. What's the deal? Or will you always be the taboo fruit of hers?