I have been with the love of my life for 13 years. In all those years I have never even thought about another woman. In the last two months I have fallen in lust with a woman at work. I can see this becoming something else. I have not had any sort of sexual contact with her but we both share blushing faces as we pass in the hallway. She is a great with conversation and I think about her often. I am still so in love with my girlfriend and it hurts my heart to be torn between the two. I just do not know what to do.
Well that's the thing. You refer to your girlfriend as the love of your life in the first sentence. There's your answer right there. I think it would be a mistake to let this go any further than a crush, afterall many smile to be nice and don't necessarily reciprocate your feelings. But don't feel bad for having a crush in the first place. It's unreasonable to think you'll never be attracted to anyone else ever again, even if you're in a happy relationship. It may be worth talking to your girlfriend about it.
I 100% agree with everything that chill penguin said, and don't really have anything to add, but I'd say to follow his advice.