As the guy left, he probably never noticed the damages, but he will later. As he left the scene, I highly doubt he will know where the damages came from as he went to tons of places. Or maybe he has a warrant or is a fugitive which is why he didn't call the police (To avoid capture). Consider yourself very lucky, and learn from your mistakes.
If someone hit your car, but you left (Even if you didn't notice the damages), you've also fleed the scene to obtain evidence and make a report.
You CAN confess, but I would simply leave it alone. But keep in mind that you should feel bad that the driver who you hit has to pay for the damages YOU created, you're supposed to be the one paying.
On the bright side, if the car you hit was expensive, you can tell yourself if this guy was able to afford an expensive car, I'm sure he's able to afford the repairs :P.
If you're a minor (Under 18 years of age), I advise you tell your parent or guardian what happened. They need to know that you hit someone because they're responsible for your actions. Yes they'll get angry, but it'll get worse when the cops tell them someone in their family hit a car and fleed the scene, and they'll realize you never told them what happened. If you're an adult (18 or over), just leave it alone and don't do it again, since you're now responsible for your actions.