Can a bank or lender contractually make me agree to lower my auto insurance deductible to $500 instead of the $1000 I have it set at right now in Ga.

If the lender has loaned you money which is secured by the vehicle, it has the right (and this is set forth in your loan agreement - the lender would not have loaned you the money if you had not agreed to this) to insist that you maintain sufficient insurance so that its interest in the car is protected.  HOWEVER, you say you have had the car for 3 months.  Normally, the lender would require proof of insurance before loaning the money, which would mean that the lender agreed to the loan even after knowing what your deductible was.  Arguably, therefore, it has waived the right, or is estopped from, insisting on the lower deductible.  HOWEVER again, if the lender insists, it will cost you far more to fight than to pay the higher premium, and may affect your credit.