Dating Crisis

Here is the story. I am a male who is 18 years old. My best friend is dating this girl. I have hung out with the two of them for the past two years. His girlfriend is now best friends with me too. I have always liked this girl but never had the courage to tell her. She has a thing for me too. The other night I wrote two poems about her. She found them and read them. She said that she really likes me too but she can’t be with me because she is already in love with my best friend and has been dating him for two years. Everyday we flirt with eachother when we know we shouldn’t. We always have a great time hanging out together. What should I do? I don’t want to break up their relationship. I would love to be dating her and I know she would date me if she wasn’t already seeing someone but I couldn’t do that to my friend.