I think what all the other have told you, is right on. You are definately being controlled and "made" to be mono to her. She sounds really controlling and demanding. To me,,,it's a situation of..what's good for the goose, is good for the gander. I think she SHOULD be happy for you, that you get to also hook up with one of her friends. Not only that, it was SHE that encouraged you, then ripped the rug out from underneath you. She obviously got jealous, but didn't expect that to happen when she encouraged you in the beginning.
I'm sorry, but I couldn't deal with the unfairness of the situation. It's one thing if you want to be mono to her, but it sounds like you also want to enjoy the fruits of being Poly, which you deserve. Age has nothing to do with it..jealousy does. If it was me, in this particular situation, I would have to fight for my rights and have girl B also in my life, after all your girlfriend should understand your wants, seeing's how she has been Poly for so long.
I have no clue as to your' "two year" wait...LOL...that one just came out of the air, in the heat of the moment. She was buying some time here, to figure out what to do. You are being taken totally advantage of here...she wants all three cakes and only wants you to have her only. She already knows in the back of her mind, that you would have both of them, if given that freedom. I think your situation sucks...ask her to become exclusive to you, with no one else involved and see how that goes....I think you already know that answer. She's just being selfish. Looks like you have two options....be mono to her for "her" two year period, to which then, and only then, you will somehow magically be "ready" to have girl B, because she told you, you were.(ya right!!) ..or secondly...go for what you want..you have one life..she introduced you to this unorthodox way of life and should accept that you are going to enjoy the fruits of Poly, right along with her. You also should be albe to contact Girl B as a Poly, without worry of guilt. Good luck to you...hope it works out in your favor...but seeing's how it's been two weeks...doesn't sound like she's going to give on her end...that's for sure...