You might as well cool the F down and think deeply whether you still love your GF or not. You said you love her but think deeply and be certain with that. If you don't really have the feeling toward her anymore, then stop make up with her so she can find another and hopefully a guy that truly love her. I am not saying that you are a bad dude or whatever, it's just that it's better to let her go and not hurt her in the future(In short, better now than then) if you really don't love her that much anymore, that is one way to showing love in my personal view. Hey, at least you are not a f****r that still trying to stay with her due to guiltiness but with no love because that's a waste of her time and her true feeling toward you.

Personally, I think your crush over your colleague is more of "lust", which is errr...short term desire drive or something like that. "Lust" goes away very quick and you might regret that afterward. Thus, you better think deeply.

You better appreciate your GF now.