My ex said she loves me but she is always with her ex

So my ex and I started to go out again. She wanted to hide our relationship though. I agreed thinking it will only be hidden for a few days. It lasted 2 months. I talked to her about this because it lasted for so long. I found out she was hiding our relationship from her ex, being complete strangers in school, but a couple at home. On our anniversary, she was with her ex the entire day. I became so pissed and told her "Thanks for a great anniversary." She broke up with me right after. The next day, she apologized and wanted to get back together. I told her we need time to ourselves because the breakup was uncalled for. She agreed and said "I love you. I want you and only you, no one else."
Her parents love me and always want me around. They invite me everywhere that they go to. They want my ex and I to get back together. They hate their daughter's ex who I mentioned earlier. They want her to stay away from him and are willing to place a restraing order on him if he doesn't keep away. Her parents give her everything she wants as long as she is away from her ex.
Days have gone by since the breakup and I find her with her ex during lunch. I don't have the opportunity to be with her besides lunch and she spends it with her ex. I talked to her about it, but she ignores me. She ignores my existence in school. When I complained, she told me, "Leave me alone, I'm not doing anything wrong. **** off." Weeks have gone by and it becomes worse. I now see her and her ex with each other before school, between classes, lunch, and after school. I'm left there to watch her ex put his arm around her. She doesn't make time for me anymore, but makes time for her ex.
She texts me everyday and I always respond, but lately since she's always with her ex, I ignore her calls and texts. Like she told me, "**** off." So I ignore her how she ignores me and go enjoy my day. She starts complaining to her and my friends saying "He's not answering to me anymore. It's making me mad. He makes me feel like crap always ignoring me making me look dumb in front of everyone." I don't understand. She wants me to stop talking to her or she'll get mad.but when I do, she becomes mad because I did. What do I do in this situation? Am I at fault?