exes (long ramble) (please help)

My first relationship came with a friend I had known for literally years. Most likely seven, but I've stopped counting as of late. We broke it off mainly because of how she views things that are important to me. (For example, I tried educating her on an active war going on when she wanted further clarification on why I responded the way I did. She then said she "didn't care until it would effect [her]."

I get it. We're teens and stuff. It literally doesn't matter, but it feels like a kind of shitty move when I clearly care about the topic and spent the time trying to educate her. Wars are a pretty big interest of mine and I've always let her talk hers out. She stopped listening to mine when they got too "mainstream" and insisted that I was going to become a popular kid and leave her.

So we eventually broke up. We stopped seeing eye-to-eye on things and split ways. Fine, whatever. It wasn't on good terms either. Almost all of our friend group went with her, since they were originally her friends anyway, and I was left with the only girl who stayed with me. (She is now my girlfriend, which is important later).

My ex (,I believe,) left me pretty alone after that. I only have one class with her and we sit on the opposite of the room. Only problem is, she has a class with my girlfriend and sits right next to her. They would talk despite my girlfriend's best interest. They had an argument after our breakup, unrelated to me, so now we collectively hate her.

My ex will talk bad about me in front of my own girlfriend. Like wtf? It's one thing to hate me, but to talk about me to someone you know I'm now dating is ROUGH. She'll say I'm victimizing her to other people even though I never talk about her anymore. I've moved on but clearly she hasn't.

Another instance of this: in the one class we share together, we have a mutual "friend". I'm not really friends with the girl, but she's involved in my new friend group so I joke around with her sometimes. We went for a walk outside as a class, and later on, my friend ended up telling me that my ex was talking bad about me AGAIN. Basically saying I'm a bad person which she knows isn't true.

She has attempted to apologize before. I cannot stand someone being mad at me without feeling physical dread, so I apologized long before we had broken up. She had made me it during her German period, claiming the reason the entire thing was written in German (I take Spanish) because her teacher wouldn't let her write in English. I later confirmed this wasn't true by one of my other friends in the class.

The entire thing made me feel stupid. She had genuinely hurt my feelings and decided to apologize by writing in a language I didn't understand and not taking it seriously. She even rolled it up into a scroll, a way we used to pass notes, and doodled on the bottom of it. I never gave her an answer.

TO MAKE A LONG STORY SHORT:

My ex keeps talking bad about me to people I know and it's starting to hurt. We didn't end on great terms so I don't know how to talk to her about it. We're going to different high schools at the end of the year and I really want to clear this whole thing up. Help?