My mother was a narcissist and we remained very close until her death although we often drove each other batty and once went 9 months without speaking. That was towards the end of her life and I now suspect it may have been partly a matter of her trying to "wean" me, emotionally, although I'm sure it was not a conscious thought on her part. I do suspect that a close familial relationship is something quite different from a voluntary one.

From what I've learned about narcissism, I would have to advise against pursuing a relationship with someone you believe may be one. There are far too many "normal" people out there to take such a high-risk chance. It may not be impossible, but I sure wouldn't bet on it. You don't have children with this person and are not currently tied to her (according to what you said), so I sincerely think you'd do best by moving on and trying to find a nice, normal person willing to accept and deal with your bi-polar illness. If you're both committed to doing that, it seems you have a far better chance of having a happy, healthy, long-term relationship. My two cents!