wow. i really can't believe i just finish reading those wall of text.
well, just ask yourself TS, is it worth to give up a three years relationship for someone you just got to know not more than 6 months.
your feelings for that colleague of yours might be temporary.

This is quite a complicated situation, I've seen people get through tight situations like this. Its a sign that you've been with your girl for too long, and its time you spice things up, or take a long holiday to rethink your views.

But always remember, whatever you do, what comes around goes around. Its a good thing that you're being truthful with your girlfriend, it quite respectable.

I've been through your situation, but the other way around. f***ing sucks, and I still have issues till now. But most importantly, you make the best of things.