Jack Canfield, co-creator of the Chicken Soup for the Soul books, has an interesting take on rejection--namely that it doesn't make sense to treat it as a type of loss.
Let's say you send your book manuscript to a publisher. The publisher sends it back. Canfield points out, you didn't have a book deal before you approached the publisher, and you still don't have one. You haven't lost a thing. And you have the freedom to go on to the next publisher, where the outcome may be different. But if it's not, you still haven't lost anything.
I think that's a really liberating way to think of rejection.
By the way, Canfield and his co-author, Mark Victor Hansen, had 130 rejections for the first Chicken Soup book! It went on to sell 8 million copies and serve as the foundation for the best-selling series of non-fiction books in history. (If you want more of Canfield's insights, see his book, The Success Principles.)