In the car business this is called being upside down. There are basically three things you can do: keep it, sell it, trade it in. You will lose the least amount of money by keeping and getting use out of the car until you have positive equity.
You could try selling the car privately. The downside is that to clear the title you would have to pay out-of-pocket the difference between the amount you owe and the amount you sell the car for. The third option is trading the car in, but the dealer will simply tack the shortfall onto the next loan, so you will likely be no better off by going this route unless the next car you buy is worth significantly less than your Passat.