Just choose who you want to date. It sounds like you are torn between 3 different people, your ex, this girl that is showing interest, and this other girl. If you really want to get back with your ex, then work on that! If you want to get with some new girl, than pursue her, who cares about what your ex thinks! Just figure out which one you want the most, if one turns you down, then there are 2 others in line!
--Ben