The easiest way to sell yourself short is to compare your work to the competition. To say that you are 5% cheaper or have one or two features that stand out–this is a formula for slightly better mediocrity.
The goal ought to be to compare yourself not to the best your peers or the competition has managed to get through a committee or down on paper, but to an unattainable, magical unicorn.
Compared to that, how are you doing?