how to handle my ex's best friend?

i am in love with my ex gf's best friend.. even that girl loves me.... but my ex does not know about it nor she knows that we talk often.... but that girl(her best friend) always feel guilty of it and sometimes avoid me and behave rudely to me.... now she told me to stay away from her as my ex came to know about our reltionship. but sumtimes she calls me for a few minutes as she cant live without talking to me.... but again she avoids me and want me to go away... I am unable to understand what to do in this situation.. plz help