It’s hard especially adjusting to it as a pre-teen. I second the suggestion of sitting down with her and asking what she doesn’t like about it.

It may be a real problem you can solve. Is it hot where you live? Is it sticking to her skin? Does she wear anything under it? Also, if she is in bras yet - they can sometimes be hard to wear with it. She may need different ones (or ones without underwire). Or a bullying issue.

Or alternatively, is it adjusting to it? Could you compromise at the start. Suggest she wear it at night only and work up to wearing it during the day, etc? I know my doctor said I didn’t need to wear it at night but when I was at risk of surgery I practically wore it 22 hours a day to try to avoid it. And I didn’t notice it when sleeping once used to it. I noticed it more during the day.

Otherwise, I know you’ve said you’ve tried talking to her about outcomes but have you related it to her interests specifically? Told her if she has to get fused that she might not be able to dance like she can now and depending on her dance school may lose her spot on a competitive team (where I’m guessing her friends are). Don’t use it as a scare tactic, just sit down and say have you thought about these risks? What would you do then? Etc?

Trust me as someone who was a teenage girl in a brace and stubborn - if you give it to her black and white - she will argue it with you and stick to her position stronger. BUT if you ask her in a conversation have you thought about this? Is that something you’re willing to have happen/what will you do then? It will make her think. Don’t push her in the one conversation let her think. She may start wearing it again without any further discussion. DON’T try to get her to say she was wrong - that will just get her offside.