How to start a relationship with your ex girlfriends best friend?

So me and my ex of a year broke up a couple of months ago. While we were dating we often hung out with one of her friends. They were very close and I became good friends with his girl ,then me and my ex broke up but me and her best friend remained good friends. I could tell even when me and my ex were dating that we had a thing for each other but recently we hooked up. I always thought it’d be nice to have a relationship with a good friend of yours but she insists that nothing more can happen because of her friend…they aren’t even very close now. I would like to pursue a relationship with this girl but i don’t know how. I know she has a thing for me but keeps saying nothing can come from it…help me out