Detach. Depersonalize. De-Ego.
The last one is a word, really. Born of the advertising industry and bred within the creative department.
When designing/concepting/creating work, you must learn to detach emotionally. Not completely, or you'd risk becoming a creative robot. But just enough to protect yourself from what is going to be years and years of critique. I think a lot of juniors coming right out of school are shocked not by getting criticism and feedback on their work but by being denied the choice to act on it. In school it's your choice what to do with the feedback, at the office not so much.
Depersonalize yourself from your work. Every critique is not a criticism of you as a person. It is so hard to not feel hurt when someone wants to change something about your creative baby. But above all else, remember this is a business. And your creative work is subject to poking and prodding all in the name of making it better and more successful for your clients. Your paying clients.
De-Ego. Again, not totally as a bit of ego makes the advertising world go 'round. But ever so slightly that your big head doesn't get in the way of your Executive Creative Director's. THEIR big head is allowed, yours isn't.