The small plastic piece should have been unscrewed from the valve body, NOT pried off. now you need a new piece for that. The stem WILL unscrew if you use 5/8" deep socket and turn it counterclockwise. You probably have the "washerless" stem and if so you need a complete new unit, but it has to be the short stem for a Roman tub and you also need a left and right hand version because you have lever handles. If that is your version, it does not have a seat in the valve body.