Why is it so hard to deal with criticism?
The reason is simple, I think: deep down, we feel that this critique is justified. This could be the case when we are not fully satisfied with our own work, or because we lack confidence. Think of a tuning fork.
When it is hit (criticism gets delivered) any tuning fork nearby will start to resonate (we react to and struggle with the critique).
The solution: be honest with yourself. When you are doing a good job, rest assured. When you could do better, improve until you are happy with the results.