In Zwift your CdA is calculated based on your weight and your height. The heavier and taller you are the greater the CdA ‘penalty’.
As for your claim of averaging 330-340w in a flat race, that seems really high even for top elite races for you not to be able to follow the main group.
So maybe you are not drafting efficiently (i.e. doing the minimum possible effort without losing contact) (?)
Maybe you are wasting too much energy in sections where you don’t have to and then get dropped in crucial moments (?)
Just speculating, it’s difficult to say without analysing a given example.