I'm surprised I haven't devoted a topic to this yet...
Get a second opinion.
If you've been reading my blog for a while, you know how much stock I put in beta-readers... a lot. Everyone knows that your novel or your short story is your baby, the infantile, literary copy of yourself. Sending it out into the world to be viewed by the literary hawks (or sharks) that drive the industry is a daunting and scary task.
To help (not guarantee) your success, maybe you should have a second (or third, fourth, seventy-eighth) pair of eyes look over your manuscript. You've been working on this particular story for only you know how long. You live and breathe it. As such, you can't look at it objectively. Scenes you poured your life into are absolutely necessary for the plot. Your words are pure genius. Your characters are deep and conflicted. Your world-building is seamless.
To you maybe.
Find someone you trust, someone that is honest. Give them your story and ask for their feedback. Nine times out of ten, they'll find mistakes you overlooked. They'll offer suggestions for revisions that will make the story better and more enjoyable.
A second opinion can confirm both your doubts and your faith in a story. By working with that second opinion, you can help yourself on the way to publication. The more you work on your story, and the more input you have, the more likely you'll produce a publishable project. So don't be afraid to find that second opinion. They may just save your novel.