You don't love your girlfriend, not really. If you, you wouldn't be getting excited about this new girl. So the right thing to do is to break up with your girlfriend. It isn't fair for her to be in a relationship with a guy who doesn't love her anymore. Do it now, before you get involved with the new girl, or get to used to feeling bad about the whole situation.
For what it's worth, dating co-workers is a dumb idea. A great relationship is worth more than just about any job, but you don't know yet if things will work out with the new girl. If things don't work out, it's going to get really awkward at work, and you might get in some trouble, depending on the company policy about dating co-workers.
Good decisions come from experience. Experience comes from bad decisions.