This sounds less like narcissism and more like two people (you and your ex) who can't seem to officially cut ties with the other, even though it's healthier and better for both of you. Clearly you are getting something out of this situation or you wouldn't continue going back and forth with her. It sounds like you've come here to get advice on how to end things with her. I think you need to tell her that you are done with this child's game and that you need to move on. Tell her you've had enough and that this situation has been sucking you dry, emotionally and mentally. Explain that for the betterment of your own well-being, you need to cut ties with her. Then, actually cut ties with her. Stop contacting her, stop answering her calls and texts, and if you can't delete/block her from social media, at least unfollow her so that she isn't popping up in your newsfeed all the time. Then, start to focus you energy on things that are going well for you. Whether that's your career, or school, or a hobby. Start spending more time with your friends, or maybe find a class you might be interested in so you can get out and meet people. The more energy you spend on creating fun, positive experiences for yourself, the less energy you will waste on this relationship.
"Caring is not an advantage."