For me, the greatest difficulty was, and still is, knowing when my manuscript is good enough to present to an agent or publisher. All writers, I think, are a bit delusional. Finding a metric for what is worthy of being read and what is simply crap is very subjective. Too a large extent, agents and publishers have the same problem. Having said that... I think I'd advise an aspiring writer to find a wide range of readers who are willing to be blunt and honest, then take the criticism given as a constructive lesson. Even the most successful writers continue to learn and progress throughout their careers. Your manuscript is not a holy grail. Be willing to toss out entire chapters, change perspectives, or whatever it takes to make a better book. Nothing is sacred.