A bad move but all's not necessarily lost. The problem is now it's all going to be framed that you like her and that it's a big deal to you. There's no mystery for her, no challenge.
But from what you've written, it could still go either way. If she already liked you, you're still in the clear. But usually girls don't know whether they really like you or not, so when you drop a bomb and force them to make a split decision, they're not all-in.
If it doesn't go well you'll have to find some way to re-frame it. You don't want her to think it's a big deal to you. Then you need to start fresh with winning her over (teasing her, showing her you have options, etc.). It's okay her knowing you find her attractive (in fact that can be a good thing), just try to steer clear of the idea you have emotional feelings for her. Don't let her think you are attached to her - you've got to be nonchalant about it.
If it seems all's lost and she puts you squarely in the friend zone, I think the better move would be to break it off completely. Then a few months later come back, but with the confidence and qualities that will make her think you are different to before so that she might reconsider. But I tend to think most of these long-term plans aren't worth it - unless you think she's the one or something, I think it's usually better to look for other girls.