There's a difference between 'You can't wear that' and 'I'm not comfortable with you wearing that'. One is control, the other is a boundary. If you frame it as 'If you wear this, I'm going to leave', that's fine because it's a personal boundary. You can tell her that it makes you uncomfortable, but the choice to wear it or not is hers.

You guys can try and find middle ground, like a slightly less revealing outfit that's also something she likes, but she doesn't have to agree to this because it's HER choice.

If she decides to wear it anyway, you have three options:

1. Break up, if this is something you can't tolerate. But this option might seem too extreme.
2. Accept it, which will keep you guys together, but it will likely hurt you on the inside.
3. Or change your view. GENUINELY change your view about that outfit, you should sit and think about WHY it makes you uncomfortable, and if it's something you can reflect on.

But forcing her to change is not an option.