Should I trade in my car?

I purchased a 2005 Toyota Corolla for $12,375.38 last jan in 08. I was able to finance the car in my name with no co signer. I've made every payment on time to this day. Before I even got the car, I had absolutely no credit whatsoever. So I decide to take the car so I could start building credit. Here's the bad part. I took the car for 72 months with an 18% apr. I know thats bad, but I got turned down by several other dealers and couldn't find a better deal on a used car. The car is in great shape for an 05 with less than 50k miles on it. But Im considering trading in the car for something else I like with a lower intrest rate too. I heard that the bank pays off what the car's worth then the rest would be added in a new loan. So I checked my car's value on the blue book and its worth roughly 7-8 grand. I'm looking into getting a new corolla or a new scion. I heard that new corolla's are around 17 grand and with a 2% apr. Should I trade in the car for another one. Please explain why or why not in detail. Thanks.