I still have feelings for my ex-girlfriend?

We're both 15 and go to different high schools. I liked her since day one of 6th grade up until the last day of 8th grade. She finally saw me as more as a friend and we started dating. Unfortunately, before school started we broke up. We still kept in touch though, we would text sometimes and hang out with other friends together. Now she has a new boyfriend and I'm hanging out with her (without her boyfriend) and our other friends. I know we're going to end up flirting tomorrow because she pretty much always do when we see each other. In some ways, I feel like she even still has a few feelings for me. I want to respect her new boyfriend though, so what should I do? Not talk to her? How can I make sure we don't do anything or flirt with each other?