I have the biggest crush on a girl, but my ex is in the way.?

Ok, I have had a crush on this girl for almost 2 years now, and I recently told her how I would like to go on a date with her. And her response was she don't want to upset my ex. they became best friends while my ex and I dated. Me and my ex dated for 5 months and that was 1 and a half years ago since the breakup. I am friends with her but it's completle platonic.. What should I do to get this girl to like me for me and not cause a problem with their friendship?