Sex...

Okay. So basically I am a 16 year old girl. With a boyfriend of 6 months. Who is ready for sex. But the problem is, I made a promise with my bestfriend. "No sex till marriage" . And she would be hurt if I went behind her back. But I am ready. And I don't know what to do. Also, what is good advice to prevent pregnancy, other than condoms and birth control?