You are unstable; your relationship is full
of arguments that are emotionally draining. You are tired of a life
that is littered with quarrelling that makes no meaning for a
relationship to be there in the first place. Guy B is an
opportunist and because he is, he is willing to carry the risk of a
rebounded target in exchange for a possible relationship with
you.
In the end, his risk probably killed him - he failed to instigate
you to move on and you realize that his role is definitely that of
a substitute.
What happened here is probably the temporal emotional retreat you
needed... the essential recess from your intense relationship. You
did break up with him, maintain that way for a couple of months
before pursuing a new relationship.
The question is: why are you turning your shoulders over and
looking back, since you have already made your decision and his as
well? The rule of thumb is that if you can't handle the implication
of 'being-friends-after-a-relationship', then avoid putting
yourself into such plight.
Do you think that a break up with your B would change anything?
Maybe that would, if A already has that hidden agenda to use that
scenario to stir your deep-seated jealousy.
I would leave two questions in your head to ponder:
(I) How would you rate the compatibility factor with A? You must
understand that Love is Love, but compatibility is another
consideration altogether. You may Love someone very much, but the
truth may be that putting you two together could conjure such
intense relationship that it may be a wiser choice NOT to be
together.
II) If your relationship is devoid of Love for B, then by all
means, cease the relationship. If there is remotely some love and
affection for B, learn to release yourself off your past and stand
by the decision you have made since you are his girlfriend and this
is probably what a girlfriend would do to secure her own
relationship's stability. As mentioned above: if you can't handle the
implication of 'being-friends-after-a-relationship', then avoid
putting yourself into such plight.
In simple, avoid complication that could leave you stranded and
lost. 
Cheers
