Should i still try or let it be?

This girl i like and me are starting to get pretty close,alas i find out shes waiting for someone that has liked her for seven years now. Theyll be dating when shes 18.(we both 15 atm)She trusts me and stuff and i really like her but now i dunno if i should still try or just let things be friendly and see how that goes,since if it goes well it could all crash and burn at 18 :/ plz help