Dear Mcsquare Pal,
I have nothing against knowing girls at pub - its
kinda fun and entertaining at times. However, when it comes to
committment base relationship - I truely doubt the chances to find
one there.
The kick about being single is simply being single - no burden of
attachment. New year is a great day to fool around causally -
however, you are probably blinded by the light airy atmosphere of
that day with recent-lost relationship. 
You dated her out and express your feelings towards her:
infatuation is obvious - The liking for acceptable physical
appearance and ambigious characteristic about her that seemed to be
ok at that point of time. 
She rejected you, yet gave you hope and went through series of
hot-cold treatement. You fell and succumb. Maybe not because she is
good at it, but rather you are unstable and unable to see through.
She has weekly boyfriend and long history of her own love. She is
not simple - like your friends suggested.
While in the relationship, she was caught being
unfaithful.
Seriously speaking, you do actually expect her to be faithful? When
a woman give her contacts to you easily - this would also happen to
others as well. No? There are bound to be one mountain taller than
the other (Yi shan hai you yi shan gao) - people whom are better
looking, richer, etc. And why not? 
Relationship is like fire. The more powerful it burns, the quicker
it dies. Kissing on second date - what makes you think she wouldn't
do that to others?
Passion dies quickly with haste.
First Law of Love states: People chose their partner, don't the
other way around. Your mind screamed a warning about her - your
heart heed its own blinded thoughts. Seriously speaking, I have
many friends who have similiar fate like you - falling for a
playgirl. Nothing of what I had said goes into their mind - well,
its ok because that would be their cosmic lesson to undergo.

If so many guys have tried and failed - its not
their problem; its with the girl herself.
She has weekly boyfriend, what makes you think you wouldn't be just
the same? Yesh, love can change attitudes and characteristics - but
based on my case studies, experience and personal opinion -
never be the first to change that. Maybe the next
relationship she has may be different/better - but the first who
tried to change that will suffer the full impact of the invisible
blow. (Karma to change her). 
Quote of the day: A love easily gotten is easily lost - a person
easily knew, its easily forgotten.
I wish you well pal. If things are getting
difficult and you need a listening ear - by all means PM or call
me.
Cheers