There is a nice book called "Loving Someone With Bipolar Disorder" - I highly recommend it.
It talks about setting goals, give specific exercises to do and work on, and give it 6 months before making a decision.
EVERY TIME I am depressed, I think I would do my boyfriend a favor by leaving him, driving out of state, destroying my identification and committing suicide. EVERY TIME. Does that make sense? Not when I am in my right mind! But when I am depressed, it is perfectly logical.
It is his reminders of how much he lives me that (sometimes) keep me going on those rough days. While you wait for a diagnosis, treatment, I encourage you to get the book. And TELL HER honestly how much she means to you, even with the mood disorder.
And you are completely right that with each mood swing, the illness gets worse! But there is still hope! I was totally disabled by it, and homebound. I still can't work a traditional job, but when I take care of myself, no one could tell there is anything different.
Remember: You cannot make her happy. Trying to make yourself a better person will not work. (Getting the book I mentioned and working TOGETHER has some hope, but she would need to be involved.)
My ex-husband would try to MAKE me better. It didn't work and the relationship turned abusive. You can't fix her, but you can use that book to learn how to work with her and make significant improvements in your lives.