I'm with st_99 here, definitely think you're over-reacting.
First off, this was NOT a date. Both of you attending an event you had independently planned to attend is not a date. It would have been a date if you drove her there, but you didn't, so I don't see why the expectations?
Talking to her 2-3 times at the gym also has nothing to do with a date, I'm not sure where you got that idea from. Unless you invited her out to the gym, she was obviously there for other reasons than talking to you.
The issue here is not what I think about this chick though, but what YOU think. Now you're saying that you lost interest in her, she lowered her value, etc yet you started a thread about her and are thinking about inviting her out. You're clearly putting a lot more effort into this than you want to show.
Once you figure that out, here's my breakdown of your night and what I woul have done: First off, when you walk in and see her dancing with a guy, there was no need to get flustered (remember, it was not a date...). I would have either walked right on the dancefloor and give her a hug (assuming the dancing with the guy was not too physical) OR try to find another girl, even a friend, and take her on the dancefloor and make my presence felt.
Second, why were you not hitting on other girls at the party? why were you sitting on a bench by yourself? Giving her a taste of her medicine would have proved to her that you're just there to have fun and are not flustered by her attitude.