When I went to university, there was a design class I had to take. He was known to be pretty tough when critiquing people's work. In the first class one of the things he told us was: "It's not personal, it's just business".
I find that it's generally helpful to keep this mindset. I try to keep in mind, it's really the result of my work (the final product) they are criticizing, it's not really me that they are trying to attack. Sure, I spent nights on something, but in the end if it looks bad, it looks bad. I can choose to feel hurt for a few seconds, over it, and make it better the next time around.
I find that it's generally helpful to keep this mindset. I try to keep in mind, it's really the result of my work (the final product) they are criticizing, it's not really me that they are trying to attack. Sure, I spent nights on something, but in the end if it looks bad, it looks bad. I can choose to feel hurt for a few seconds, over it, and make it better the next time around.