Damn Fabio, you been workin da ladies lately? :P
If I had done that, I would probably hate myself. From that point on I would realize that the relationship I've had is over and it's all my fault. On one hand, I tell her what I've done. This brings on a spiral of emotions and she'll never be able to trust me again and even if we do stay together a little longer, I feel like she will always feel a little anger towards that and may find herself in a similar situation to mine and simply use my actions as a justifier to do the same. All the hate and distrust and wonder will eventually cause the relationship to be incredibly unstable and ultimately end it.
On the other hand, if I keep it to myself, the guilt is overwhelming. And the fact that there's no way she can find out just makes it worse. It means that I've done something without consequence and, if I push it deep enough down to forget about it, I might just start accepting it and continue to do what I've done. My feelings would only grow shallower for the girl I'm in a relationship with. And to keep her hidden from that? That's just unfair. I'd want to know if my significant other's feelings for me were leaving. Why keep someone on a string like that, infatuated and believing that everything is okay, when it's not. And inevitably, if it's not, it will become apparent. In this particular matter I'm speaking from experience, it just goes downhill from there.
If there are no consequences to doing so, then you'll just keep doing it. You gotta learn from your mistakes. That's why I'd fess up and tell her the truth. I'd be the one responsible for the end of our relationship but hey, that's what happened and this is what's going to happen. You just gotta know how to come up from it and move on. Just hope that your ex doesn't tell anyone (wishful thinking) cause no other girls will be quick to get on the relationship level with you.
Thus, the birth of a player.