Love's Mathematics: 1+1=0
Your situation is slowly becoming like that, good thing you still know your do's and dont's but you on the verge of surpassing those.
I think you should be honest to both parties and talk to them about it, if your bf truly loves you he should understand you and as well as your bestfriend...
it is a risk that you should think about...
most cases it one would really have to leave your life but at least your being honest to both your bf and bestfriend...
If your bestfriend is really your friend, he should understand...
I've been in the same situation before, where I was in the place of your bestfriend...similiar situation, she talked to both of us, her LDR bf and me, I gave way to the bf...they're married right now....
Just be honest to yourself and both parties, in any relationship, failure to talk and communicate with your loved ones are mostly the mistakes we commit that end up breaking the relationship...
Hope you figure it out!