I’m seeing a little of what I’d describe as toxic monogamy in this question. That’s not to imply a monogamous relationship isn’t right for you; it’s what most people want, so just statistically, it’s probably what you and your girlfriend want.
What I mean by toxic is that the belief that it’s unacceptable to be friends with an ex or that you can’t be in a relationship if your feelings for a former partner aren’t completely gone can poison relationships. People are usually more complicated than that, and while both of those things sometimes lead to problems, they don’t have to. Talking to your partner about their feelings surrounding the issue and agreeing on boundaries that are acceptable to both of you will prevent a lot of problems. That brings us to…
she asked me about our relationship and I told her to stop talking about it
I don’t usually like to speak in absolutes when it comes to human interactions because different people are different. I’ll make an exception here though. This was the wrong answer if you want to have a relationship with her.
It’s still the wrong answer, and that’s where you left things. If you don’t want it to end there, your best chance is to tell her you know that was a mistake, apologize, and offer to talk about your relationship as much as she wants to.
It’s OK not to know what you want. It’s especially OK when you’re 18 and in high school. It’s OK to choose an option now and change your mind later (though if you choose no, she probably won’t be available to you later if you change your mind).
It’s even OK to tell her that you don’t want to talk about your relationship, but she’s probably going to take that as you not wanting to be in a relationship with her. If you don’t want her to take it that way, then you need to talk with her about your relationship even if the thing you have to say is you’re unsure. Of course there’s a good chance she’ll respond unfavorably to your uncertainty; most people don’t like being a second choice or a backup plan.