Buddy, I think it is very obvious that you have yourself a handful of trouble that you want to hang onto because you have great sex and you appear to view her as some kind of trophy that would ordinarily be beyond your reach (in your mind anyway).
From what you describe there is a whole lot going on beyond any possible issues of her being bipolar or not - such as past history that probably needs to be addressed by someone much more qualified than yourself.
Beyond the physical and ego desires, you seem to be a caring guy - though it is hard to say how much is caring and how much is wanting to make your hot trophy more livable. At any rate, you have tried to get her to seek help and she has refused. Your friends and her ex's all see what you can't while your blood is mostly between your legs instead of between your ears.
Trust me, you can't fix her. Maybe someday she will seek the help she needs and take a good look at what she needs to fix in herself. Until that happens, you have done what you can and you had best look at what kind of fix and misery you may be putting yourself in.
Life is simpley too precious and short my friend, to spend even a day of it arguing and being miserable and such and dang sure too short to spend in misery trying to fix someone who refuses to be fixed. So, be happy for the good sex and ego boost or whatever you have enjoyed thus far, wish her the best and repeat your plea for her to get help, and then have a good talk with "Little Feddie" or whatever you call your whanger and convince it that taint no poohninnie worth that price and it is time for the both of you to move on!
You know what they say about the reason that guys give their, umm, male units names, don't you? It's because we don't want a stranger making so many important decisions for us!