How can I handle my best friend dating my ex?

okay, so me and my best friend are fighting because of a guy. everyone knows how much he always meant to me and how much i loved him, even after we went out, i still had reallly strong feelings for him. but when he asked my best friend out like 3 days ago, she didnt even like him but now she does and theyre going out. what bothers me is i still have a lot of feelings for him which i dont think will ever go away but i cant tell her that because i guess now she likes him and i think that would be really wrong if i got in the way of them. but she kinda knows i still really like him and i keep telling her i dont so she wont get mad at me but it just sucks. whats really weird is that she thinks he likes me too becuase he kinda came over yesterday to hang out before we all went to this fair thing. and like she got really mad that we were alone but she knows i would never do anythng and he probly wouldve went over her house but she had a soccer game and he said it was boring at his house. i just dont get it and i dont want her to be mad at me. i reallly like him and i know thats wrong on so many levels but if i could change it i would in a heartbeat. i just want her to be happy. but i dont wanna lose him. i am so confused! what do i do??