You were wrong to be disloyal to your best friend in this way. How would you feel if she'd slept with your boyfriend? This boy doesn't have much character if he cheats on his girlfriend with her best friend- and he's most likely just using you for sex. Had he really cared about you, he would've ended it with her first- and been more honest.

If you're determined to be with him, then you need to be honest and tell your best friend the truth and hope she forgives you. If you continue to cheat with him behind her back, she WILL find out for sure and you WILL LOSE her friendship- possibly forever. So, tell her the truth and then this boy's actions will let you know what he's after. If he continues with you, there may be hope for the relationship- as long as he doesn't cheat on you as he cheated on her. If he backs away from you after you've told her the truth, then you'll know he doesn't care& only wanted sex all along. Had you insisted on his being honest with yr best friend and breaking off with her before allowing him to sleep with you, you never would've found yourself in this position. I hope you can work it out.

Dear lonely heart,
what you are doing with your best friends boyfriend behind her back isn't ok.
you gotta tell her, because what you are doing to her now, is the most betrayal.
My advice is, that you and her boyfriend tell her together. Because I think she
actually doesn't want to have a boyfriend, who loves her no more.
Maybe if you say you're sorry, and why you have done it, she may understand - I mean,
she's your best friend, she knows you.
If you continue cheating on your best friend, THEN you'll lose her.
Though you want to be together with her boyfriend, I would wait before the
wound in her heart has healed, so that you don't hurt her, more than you've already done.
It's a complicated situation, but I hope, with my advice, you can
make it right again.
Good luck!

CeLi