gm financial Won't work with you!! arlington, Texas

Either you bought a cheap Semi-Truck, had horrible credit and bought way above your means, or you are not telling the entire story.   Even taking your low amount of $800 a month over 6 years that is $57,600 in payments.  With the remaining $13,000 they say you owe that brings you to a grand total of $70,600. 

So my money is on the later two options.

It would take a 19% interest rate(very high) and 35K loan to even approach a monthly payment of $800, it would take a 50K loan at a good rate(5%) to hit the same range.  Then almost no matter what type of loan you have(some balloon loans wouldn't qualify), if you have made all of your payments On-Time and for the full amount owed each time.  At the end of the term..your car will be paid off. 

Next, car payments are a "fixed" amount, and I doubt you have paid extra(as if you had it would be paid off sooner).  So with those two Items I would bet that at least a few times during your loan you were late, and possibly even to the point where they already had to work with you to keep your car from getting repossessed.  Or by chance if it was repossessed they worked with you to get it back.

So what's the whole story?