I have a relationship with my best friend but she doesn't accept it what to do?

Hi guys this is my first time here. Btw I ‘m a girl and I’m 18, recently I ve started dating a girl but she is my best friend. I have always loved her and it’s a dream having this relationship with her but the problem is that she isn t gay or bi and for this reason she does’t accept me as her girlfriend and I’m really scared that soon she will leave me because she wants a man… what do I have to do?