welcome to sgforums, nice nick btw.
no this isn't a very new problem. it's actually pretty damn
common.
let's tackle the obvious problem. u have feelings for an attached
girl. obviously u've already screwed up by letting urself fall for
such a person. no worries, i've done it b4 too, not berating u or
anything.
generally speaking u have to get out. it's pretty simple really. if
she stays with her guy, u will continually feel like sh[/i]it. if
she leaves the guy for you, you're gonna be filled with insecurity.
u know the usual "if she can do it to him, she can do it to me"
crap? well that and more will come to ur mind. either way, u
LOSE.
the "compromise" to this situation would be to keep her at a
"decent" distance. now this distance is gonna be hard to control.
of course u both still wanna be in touch and "be friends" and all
that, but u have to be resolute that u will not harbour thoughts in
any other way than friendship. some pple can do it, some can't. if
u can't, cut all ties. if u can, then i hope u are strong enuff.
ways to tell if u are on the right path is that u start looking for
other suitable partners, opening ur heart to other opportunities
that come knocking, and even start chasing other pple. for the
right reasons of course. you're after all single, and should
exercise that right. why be single in reality but be attached to a
(some say, immoral) hope?
now the not so obvious problem. how do u know u LOVE her? if u can
ascertain that it isn't love that u are feeling (which i highly
suspect) then u will have a more logical and rational frame of mind
to make urself feel better. if u keep telling urself u love her,
u're not gonna get yourself out of the hole. are u sure u love her?
how much time have u spent with her? how comfortable is she with u
despite having a bf, which would gnaw at her conscience if she does
have feelings for u too? with all these hanging over ur heads, how
well can u all get to know each other to determine love?
i believe love cannot blossom under such circumtances. as if love
blossoming over FAVOURABLE conditions these days isn't hard enuff.
now u have unfavourable conditions.. even worse rite?
i speak from experiece. i have fallen for attached girls. girls
have fallen for me while i've been attached. i have been with girls
that were attached and broke it off becos of me. i have been with a
girl that broke off with me becos of another guy. none of these
experiences were very sweet. 
no one
wins....
good luck, and come back often and post more. u may find ur gf
here.. many girls here.. many single too! 
