Someone CAME into your heart and seized the throne
that was meant for your gf.
This is a truth.
And when this
someone actually had the POWER to do so... effectively, your Love
for your gf is waning.
And when you have reflected and thought that the reason why you are
staying onto the relationship is NOT out of Love... effectively,
you have also lost the core reason why you should remain in
it.
P.S: The other girl's existence is a mirror onto your own fragile
relationship - it reflects every corner of your love in details...
that Love may be missing right from the beginning. 
Cheers
