HELP!!!!!! I LOVE MY BEST FRIEND'S BOYFRIEND!?

Ok so for the last 2 years i have liked this guy named Alex and at first it started out as a lil crush but now im totally in love with him! hes sweet,caring,romantic,funny,and knows how to have fun...but the thing is my best friend has had a crush on him for 3 years (to me time dosnt matter) we both knew we liked him and we were totally fine with it, eventually he became our best guy friend and i decided to tell him of my crush, he totally rejected me! saying that if i wasnt his friend he would of gone out with me(bull*hit), but i still like him, now one day after my best friends 15's he texted me saying, hey i think i fell in love with Betty,i was crushed and happy at the same time but it hurt sooo bad and now hes gonna ask her out next week and my best friend asked me to come with her! i cant forget about him what should i do?! she already knows and she feels bad but shes still gonna date him the heartache hurts so much i have to cry myself to sleep (btw hes 1 and a half years older than me just like my best friend) what should i do? i love him but i dont want to lose my best friend