You have a bad blower motor resistor. When they fail, you'll start dropping lower speeds until finally have only high left. It is located in the heater box next to the blower motor under the dash on the passenger side of the car. The blower motor will have to be removed to access the resistor. There are two different ones for your car. One for standard single zone a/c (AC-Delco part number 15-8599), and one for dual zone a/c (AC-Delco part number 15-80571).