Zuzzlefish (weirdly cool username, btw),
Your relationship doesn't sound like one that brings you much satisfaction, happiness, or fulfillment. Every relationship has its ups and downs, but I always say one needs to look at which direction the scale is tipped most often. Is it mostly sunshine and rainbows with a few dark clouds every now and then, with good communication and mutual efforts to resolve issues, or is it mostly thunderstorms and drama, with lots of arguments and hard work to reach a good place between you?
If, at only eight months, which isn't long at all, you two have had a recurring conflict over your ideals for relationships, I would say that does not bode well for you. You have felt attractions to other people, but attractions don't always need to be acted upon, whether one is poly, in a relationship, or not. However, you have also considered ending your relationship with her, yet you stick around because of guilt and feelings of attachment. You get confused and don't know how to create for yourself the things you need to feel fulfilled and happy. She is often negative, depressed, and perhaps doesn't manage her borderline personality disorder very well. You sound like a stand-up guy. You try not to judge and want to be understanding, but you feel emotionally and energetically drained much of the time. You hold out because you hope for more of the uplifting, refreshing, "high times" you know are possible. You sound a bit like an addict, or co-dependent to me.
My take on it is: get off that "beautiful yet ugly rollercoaster." Walk away, it is not good for you. You deserve better. But I would start reading up on co-dependency, if I were you (look for Co-Dependent No More by Melody Beattie), so that you are not feeling stuck like this in the future.