Well, this is sort of a tricky situation, isn't it? People could give you varying advice on this, and it comes down to your personal morals. I'm going to tell you what I would do in your situation, and you can do with that information what you will.
I would tell your current girlfriend. I feel like the woman you love has every right to know that you slept with another person. Yes, she may break up with you. That's one of the risks of cheating. However, you seem to feel pretty guilty about this, and if you don't tell her, you might feel bad about this for the rest of y'all's relationship.
If you're concerned about things like this happening again, you could speak to a counselor about impulse control.