Hey there, I'm really sorry about your situation. I'm in a similar one, let's just say that. I agree with everyone who said communication is really important. An honest relationship is always the best. Talking to your boyfriend about it, and then your best friend, is really going to help.
Thing is, if I talked to my boyfriend about it, it might end up in a breakup and I don't know if I'd be able to deal with that. And in my case, it's not my best friend, just a close friend. Well she WAS a close friend...
If she really truly is your best friend, she'll understand if you talk to her about it. In fact, if she truly was you best friend, I don't think she'd do it at all. I don't understand anybody who would do that, even if it was just a friend.
It's sad really. Talk to your best friend about it first. If that doesn't work, go to your boyfriend and bawl the truth out because it will help. You know him well and you know how he's going to react so it's totally your decision.
I hope everything goes well. You seem like a really nice girl who doesn't deserve this.