HELP ! I Think I am about to be screwed over into another used car trap!?

Ok so...my dealership is in Memphis, TN and I live about a hour away in Ark. So I have this 2000 Chevy Malibu that i got out of desperation bc i needed a car and it had 140K on it at the time(July 07'). I have had to have the rotors n pads changed 3 times in a 6 month period ,and within 4 months of purchase it had a blown head gasket(possibly could have been more)...had that fixed, and before that had to replace a fan belt, and here we are again a year later needing a new motor,

So I call and tell them, hey I want to be put in another car because i feel it isn't even worth replacing with 173k on it, its ridiculous. So I come to the lot thinking I'm going to get whatever i want, but instead, they tell me that since my payoff for the vehicle is to large, I am limited to 2 vehicles(a 00' Pontiac Van and a 02'Taurus) The van i saw, and the Taurus was in the shop getting serviced the same day i was at the lot, never saw it, never asked how many miles were on it because I was already pissed off and I was not interested in neither vehicle.

But here is the big catch, I am currently on an 18 month agreement with them where i keep it that long to be elgible for trade, but they are wanting me to renew it for 36 months and rollover the 6K I owe for the Malibu. I feel this is not fair and I don't feel right about it. I'm scared to sign these papers for something I really don't want, and I feel once they sell me a lemon, they will be more than happy to do it again. Suggestions ?