I am always amazed at the number of rejection letters I receive in comparison with acceptance letters. Like most artists, I receive far more rejection letters than acceptance letters. The trick is, to not take it personally and to keep looking for new opportunities. Creative people must have an extremely thick skin. If I took every rejection personally, I would end up living in a padded cell eating pH balanced food without any sugar. It is a good thing I don't take them personally!
The truth of the matter is that every opportunity has hundreds and sometimes thousands of hopeful participants. Most opportunities can only accept a couple hundred at the most. Every judge, curator, gallery, and collector has their own tastes as well as likes and dislikes when it comes to art. Just because you get rejected, does not mean you aren't creating good art, it simply means that whoever was making the decision liked another person's art better.
I honestly believe in something that a well respected (and collected) artist once told me. He said, "There is a buyer for every painting. You have to be patient. All it takes is one."
One is all it takes. One buyer, one judge that likes your work, one curator that enjoys your style, or one gallery that finds your work unique in order for you to get a letter of acceptance.