"I am so confused, I keep going back and forth and soon neither one is going to want me..."
You can't possibly make a decision while you're in this frame of mind so my advice is to not change anything until you know one way or the other. If you force yourself to decide for one or the other now, you're leaving yourself open to future second guessing your decision and I wouldn't be in that position.
It sounds to me as if you're in a kind of open relationship (he knows about her and she knows about him?) so if this is true there's a possibility that you won't make the decision, one of them will. So it may come down to the last man standing, er better make that the last person standing.
I don't have a problem with same sex relationships but generally speaking, bisexual people are romantic with their opposite (men and women) but they can enjoy sex with each other (girl and girl or guy and guy). so is it possible that the erotic part of your bisexual girlfriend is what attracts you the most?