So I dated an emotionally unstable girl who has problems, a very high ego and intelligence for a few months. On our 3rd month, she learned that her ex started a new relationship. Even though she didn't love him at all, because of her ego, she cheated on me with him, then dumped me and got back with him. I was feeling like shit so I wanted to have revenge. I brought her panties which she forgot at my place (don't ask how that happened) to school, found them sitting together and threw it on her face saying "here,you forgot this". Somebody recorded this and the video spread all over the school, making her unable to show her face again. Then I met with the guy and we talked for a while, showed each other her texts and learned that she has been playing on both of us for a while. 2 Days later, she started calling me. Then came to my place. We had sex, drank some alcohol and suddenly, we were veeery romantic. So we started dating again, she was so full in love this time. But I couldn't trust her (She promised me she'd never see that guy ever again, and that she could face him and tell him she that the one she loves is me, so I was relaxed) and she couldn't get over the cruel thing I have done to her. Still, it wasn't bad. But slowly, she got worse and worse and started not feeling excited while she's with me. Eventually, we broke up, but continued talking the whole day as we were doing before. She said she won't be seeing anybody for a while. But 2 days later, she confessed that SHE WANTED TO GET BACK TO HER EX, AGAIN. She doesn't love nor want him, but as I said before, she has problems, and she always goes for the worst thing (worst personality I've seen so far). I told her she'd lose me in that case, so she cried for a while, saying she can't go on without me. Now, she's about to do what she said. And I don't know what to do. Feeling the same thing again is just too much, and I can't just simply leave her. I have an important exam on Sunday but all I can think about is her, and this gives me nausea. This actions of her makes me feel like she never loved me at all. So any idea how can I get over this?

tl;dr: She cheated. I got revenge. We got together, and broke-up again. Now she's returning to the guy she cheated me on with, but also trying to keep me on her side. And I can't get over with it.