The sad plight you have landed yourself into
is probably because there's an aperture in your relationship.
Perhaps you reckon your relationship is unfulfilling or that your
relationship is too calm for its own good and subconsciously, you
'summon' waves to jazz up your love life.
You may think that sounded illogical, but people does that sort of
things all the time.
It's because our experience in love is never stagnant. In fact, we
face challenges every moment of our life.
You may think that this woman represents non-committed/somewhat
free sexual relationship (AOS will likely attribute her as a
Siren), but the truth is that her presence presents a cosmic test,
which you failed utterly - judged by the strength of your love
towards your girlfriend and the relationship. The flesh may be
weak, but surely the flesh is the lowest level of all our higher
senses. If the
mind, heart and soul can be easily dominated by our animalistic
instinct (Mars), which is the lowest level in our human
consciousness 'chain of command', then the path you walk will
surely lead to misery and sorrow.
To understand the driving force behind your decision to betray your
girlfriend, you got to first understand whether was it a push or a
pull factor. The former represents a fissure in the relationship,
while employing an avoidance-defensive mechanism in respond to this
crack and the latter represents flaccid self principles devoid of
wisdom.
Eventually, you may just decide that your 'mistress' is the woman
you are seeking all these while. But as you reach for her heart and
find nothing but illusions, it may just dawn on you that you are
just become a pawn of own karmic debts.
I pity your woman, not because you are a man who went astray, but
rather, you betrayed her in both levels - physically and
emotionally. A woman might be able to source some remote reason/s
to salvage a relationship if a man loses one aspect among the two,
but certainly not both - because it makes no meaning to retain a
man who desire to flee and a man like you supply her no reason to
stay.
Perhaps you might want to rethink your situation; albeit holding
onto your girlfriend may be the best of both world for you, but
such selfish grasp onto her may end up tearing your guilt wounds
wider.
I can safety say that chances are, you won't be able to convert
that Siren to settle for you exclusively. Your contract with her
never had that clause and you are probably just like the many other
guys she had sexual fling with. Therefore, what makes you think
that just because she got into bed with you, it implies that you
have power over her?
P.S: Man are often lost when the tables are turned against them, as
they become the prey, completely subdued a woman with voracious
sexual appetite. 
Cheers
