I have written my first YA novel and have tried to get it published. I have received quite a few rejection letters. I have also tried to get an agent, which I have read is much harder than getting a publisher. I thought I did the hard part writing the novel ... lol. My question is how do I know if my book is any good? I've read that the agents and publishers get such a volume of work sent to them that the slightest thing can make them not read it. I really enjoyed writing this story but I would like to know if it is worth pursuing other publishers or agents.
In short: you don't know. The reason for this is that everything to do with art - books, music, paintings, whatevs - is highly subjective. One woman's trash is another woman's treasure etc etc.
Here's what I tell my clients when we're looking for a publisher: 'You only need one. It's nice if there's more than one, but you only need one.' Of course, the trick is finding that one publisher, or agent, who will love your work. And maybe, just maybe, this isn't the novel that's going to make that happen. Or maybe it is and you just need to keep looking.
You don't mention whether or not you are writing other things. If you're not, I advise that you do. You shouldn't pin your hopes on just the one manuscript - first, because that's a lot of pressure on one li'l manuscript, and second, because it's rarely the case that it's a novelist's first ever manuscript that gets published. For every published 'debut novel', that novelist has a few in a drawer somewhere.
What you do mention is that you've worked out that the hard part is not just writing the novel, and that is correct, because it is not the novel's job to sit in the dark with no friends. Stories need an audience, and finding an audience is always the hardest of the parts. It takes patience and perseverance, guts and determination. This endeavour you've embarked on is not for the faint of heart. Your decision as to whether or not to continue depends strongly on how much you feel that your story should find that audience.