"The writer must believe that what he is doing is the most important thing in the world. And he must hold to this illusion even when he knows it is not true."
- John Steinbeck
Believe that you can make it as a writer. If you don't have this confidence, it is will be difficult for you to find the impetus to send work to publishers and continue to keep writing diligently. I have had the fortune of encouragement from many of my friends, teachers, and family. But without these, you still have what you know inside: that you can become a great writer, despite what publishers and colleagues are saying.
Hold firm to your confidence - not ego - but a self-respect for your potential.