The best advice in this situation is assume that it will not work out. I think a lot of the "nothing comes out right" is because you're nervous now. When you were still dating "a" you saw it as impossible to be with her, so there was no pressure. The problem is the pressure is on now because you realize you now have something to lose (because you actually have a chance to be with her). In my opinion the best way is to just assume that it won't work out, but hope for the best. It certainly seems like thats the problem at this point. As for the whole "she dated a guy to get to me", did it not work? Would you still be with "a" if she hadn't? Maybe you should be greatful that she affected change.