You should not have told your roommate she could invite her.

You should politely let the roommate know that you've reconsidered and think it would be best the ex not attend. If she asks why, be honest. Don't be bitter, but be straight forward. "The break up was hard. She cheated on me quite a bit and I'm still not completely over the things that happened."

If the ex still shows up, simply remove yourself from the situation.