How do I deal with falling in love with my best friend?

So my best guy friend liked me and I didn't feel the same way. So, with my permission of course, he started dating my best friend. But me and the guy still talked a lot, more than him and his girlfriend even. And after getting to know him even better than before, I began to develop feelings for him. SO this week we admitted we liked each other and if he wasn't dating her, he'd date me and he still has feelings for me. Now I didn't realize how much I liked him until she and him were talking on facebook and he didn't text me like he said he would. I was really pissed and jealous. I finally admitted to him that I really really like him and I think our flirting and joking is wrong and not fair to her and its breaking my heart. I also told him we shouldn't be friends. But he said he couldn't do that. So I have no idea what to do because my heart is breaking, and I can't tell his girlfriend, my best friend, and we both have no idea what to do. Suggestions? Our relationship is getting way to complicated and I'm afraid something will happen and I wouldn't try and stop it. And its not fair to her because I know she would want to know and its not fair to me because my heart is breaking. I'm not mad at him but I just need some advice on what to do. Since not being friends isn't an option what should I do? And I don't know how I'd tell her. So should I and how? And what I should do about him? How do we untangle this mess of a relationship?