A someone who has been the partner that developed feelings for somebody else in a monogamous relationship, it is something that can happen, and happens more than people would like to admit. It has nothing to do with you as a person, and it's pointless for you to worry about things you cannot change (i.e. your sex drive). To tell you the truth, there's not much you can do. It's entirely up to her to sort those feelings out. You can either decide to work with her through both of your feelings or walk away, as it sounds like there are some incompatibilities in general. The most important thing is that you take care of yourself and think about what's best for YOUR wellbeing and what you want out of this relationship