Disappointments are part of the journey too. I get disappointed when a magazine turns me down, this is quite painful sometimes. But in the other hand, it helps us to be humble. The same magazines that turned me down before are now asking for my photos. If I didn't have these turn downs I wouldn't have the tenacity to progress. I knew that something was wrong, something needed to reconsidered.
Sometimes I see colleagues that become mad if they are not accepted or requested, but they always blame others, never give themselves a chance to revaluate their work or methods. Lesson number two: you have to be humble enough to accept critics and deal with them. We never stop learning. If you think you know it all, retire, because there will be no market for you anymore, the world is not going to adapt for your benefit.