Have a girlfriend but im in love with my best friend..?

me and my best friend have been in love for like 5 years. but both pretending we dont know it. we havnt dated because i feel were gonna get married.. and i dont know if im ready to think bout that. ive had a girlfriend for 6months and shes a great friend. but idt she'll be anything more than that. but i dont know if i can break up with her. cause idk.. i dont know what to do. some advice? entering my senior year