Unfortunately, rejection is a huge part of the literary business. The thing to remember is that it's all about finding the right match. When an agent says 'I'm sorry but I'm just not right for this project', it's not a cop out, it's actually true. Anyone who is less than enthusiastic about your work isn't right for you. Just as often you'll hear, 'it's just not what I'm looking for', or 'it didn't grab my attention like I'd hoped it would'. So what are you to do?
First, don't give up. Use their rejection to inspire you to better your story. The last thing you should do is shelve it because fifty people or more said no. If you got a lot of the first reason then you most likely just need to research those you're sending it to a little more intensely. At the same time though, make sure you have edited your work to the very best of your ability. The submission stage is no time for grammar errors or bad sentence structuring. Unfortunately, editors don't expect to edit as much as they once did. Besides, a really polished piece of work makes you look like a pro and goes a long way in impressing both an agent and an editor. As a first time writer the odds are already stacked against you, but if you look like you know what you're doing and appear dedicated and proud of your work, you'll get a closer look from them.
Outline your story. You'll be surprised what kind of issues a simple outline will reveal to you. I was not a believer in outlines until I was forced to do this. Now, I won't write a book without one. It helps reveal any plot errors, repetitive issues, or excessive slow or fast areas. Doing this may inspire you to change, delete, or add entire chapters. It will surprise you!
In short, use rejection as a reason to go back over your book and improve it. I don't necessarily recommend going back and working on your book after every rejection letter, that would be a mistake. But once you've done a round of submissions, say thirty or so, with no luck, then it's time. Thirty is certainly by no means a form you have to follow, it could be fifty or sixty, that part depends on the number of people you feel might be really interested in your story.
So what if it's perfect like it is? Just like everything in life, there is always room for improvement. If we stop improving, we stop growing as artists and that would be a tragedy.