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On what date was the original contract made?
If the contract was signed on 4/12/08, then the first payment would be due 5/12/08, unless the first month was prorated in the contract. That said, if the seller received down payment for the vehicle that was more than the value of the vehicle's use for the time in your possession, plus recovery and repair costs, if applicable, then you should be enttled to any amount over that refunded.
The seller probably doesn't trust that you will pay, but if you take her to court, it doesn't mean that you can get the car back, if you violated the security agreement.
Your best move is to try to negotiate, in my opinion -- at least based on the facts you've posted so far.