Dealing with rejection letters is something writers and illustrators face on a regular basis. For truly sage advice on this matter visit: http://editorialanonymous.blogspot.com.au/search/label/The%208%20Rules%20of%20Rejections. "Most rejection letters mean nothing. Nothing. (Except that you can cross that publisher/agent off the list.) The only possible exception to that rule is specific, constructive criticism. If it is not specific, it means nothing. If it is not constructive, it means nothing. If it is not criticism, it means nothing."
This next article is a brilliant list of common pitfalls that writers and illustrators face in developing their creative careers: http://www.artsbusinessinstitute.org/blog/10-mistakes-that-stop-your-creative-business-in-its-tracks/. Take a few moments to review your current creative practice against these 10 very common mistakes: No plan; Lack of persistence; Working in isolation; Not understanding your audience; Being unclear on your message; Playing it Safe; Not understanding your sales cycle; Having little or no online presence; Lack of marketing strategy; Failure to follow up or follow through. Choose one to work on, and see how big a difference it makes.