I just screwed up big time?

She knew exactly what she was doing and she knew you wouldn't say no. You were not in control. If you want to get with her, just be nice, but not flirty. If she talks about doing anything (sexual) again, tell her as soon as she dumps her boyfriend, you'll consider it. Say it with a straight face like you're dead serious. And make SURE you confirm they are split up before you get with her.