I'm guessing you're either in the same school, or know where she lives. I suggest you try this:
1) Call her up, and ask to meet her. Don't tell her what it's about, chances are she'll get curious and come, even without an explanation on your part.
2) When you meet, sit her down and talk to her. Tell her something like, "Listen, I know breaking up is hard, but I've moved on, and you should too." And top it off with a, "You broke up with me, remember? So I don't know what the big deal is if i'm seeing other women."
3) Hopefully she got the message, and you two part on good terms. If she tells you that she still has feelings for you, tell her the same, but DO NOT get back together with her. She dumped you before (causing all this shit), and chances are she'll do it again.
If this doesn't work, then you've got a psycho bitch on your hands, and I suggest moving out of town, because things tend to get ugly when chicks don't leave you the fuck alone. Hope this helped.
EDIT: The answer to your question, no you are not wrong. She broke up with you.