I think a lack of belief in one's own work can be the biggest obstacle for a young author to overcome (or indeed a new author of any age). If you don't have an agent your work is likely to be rejected many times, and even if you do have an agent, it can still be difficult to crack into the market. The thing to remember is that people who persist are the ones who make it in the end. There are plenty of famous authors who had their manuscripts rejected many times before their works were finally published. I have even been told Walt Disney was rejected over 200 - 300 times by the banks, when he was trying to get funds for his first Mickey Mouse cartoon.
Try and try again and always believe in yourself.