Hm, you present something to really think about it. Let me give my two cents. Take it with a grain of salt.
I believe that it's never time to quit when writing is what you truly enjoy. My advice: Don't throw in the towel. The towel doesn't exist. There never was a towel to begin with because there never was the option of giving up. When you wrote this series, you wrote with the intent of getting published. There is no giving up now! Keep pushing ahead! You have the entire YA community on your side, rooting you on!
To quote TTGL, "Don't believe in yourself. Believe in me, who believes in you."
I just finished my first novel and am currently running it through the editing process. I'm only fifteen. Can you imagine how difficult it will be to get an agent to take me seriously? But that doesn't mean I won't try. I'll show them just as you'll show them. I think you'll prove yourself wrong. I believe in you!
I think we have to remember that everyone has their own personal reasons for writing. And just as this is the case, they have their own personal reasons for "giving up." For some publication is a goal so they can finally get paid for doing what they love. I see no reason writers should not make this a goal. It's really hard to continue to query. I STILL get rejections and have an agent now. I sometimes forget and for a brief time get upset at the rejection but them remember I have Mark. But then get re-upset because I feel like -- wait. How come you don't like my book? I have an agent now. Don't you see how wonderful I am? NOT.
Truth is, I don't feel all that much better now that I have an agent. Now my fate is in his hands as much as mine. It's kind of surreal. I met him in person a few weeks ago and I nearly broke his back hugging him. I think he must think I'm a nut. but I wasn't necessarily hugging him--I was hugging "my agent" or the idea of having one. I am one step closer to becoming published. BUT WAIT. I'm not. Now I have to wait for an editor to like it. And what are the odds of that? I'm hearing more and more from my agented friends that it is harder and harder to sell books in this climate--especialluy one with a subject matter like mine--which is half the reason it was rejected so much in the first place.
I'm hearing lots of agents sending clients back to the drawing board for a second book because the first one didn't sell after months of editorial feedback and rejections from publishers. The one good thing is, once you go through that, you CAN sell a book on spec. You don't necessarily have to write the entire book FIRST then sub. But I digress.
So, I'm saying all that to say this. I've waited 2 1/2 years to secure an agent. During that time, I revised my book, founded #YALITCHAT and then YALITCHAT.ORG. I wrote 4 other books which are in varaious stages of completion. I started an editorial and critique business which helps pay the bills and added ghostwriting to that. I blog about writing and the business of YA. So, even if I'm not writing my books on a daily basis, I'm writing and getting paid for it. I'm helping others to write better and reach their full potential as writers. I'm learning how to be the best writer I can be. All of this has brought me to where I am today. Where is that you ask? Feeling 100000000x better about the journey that has taken me THIS long to find an agent because I found the agent that was right for my book and my personality. See how that works? And I was in a position where I did not have to query this agent, I was on the phone and I simply asked him to read my full maunscript and he agreed to. If I had never been founder of YALITCHAT and done all this other stuff, he and I would never have been on the phone.
You can create your own destiny. You only have to see the path, believe in yourself and do it. You do not have to wait around for responses to queries. They are not a measure of your worth, how good your book is or not or what your value is to the literary landscape. I promise you this. It is a matter of the market, a person or a few people's perception of what can or cannot sell in the current landscape OR what they are personally capable of selling and sometimes personal taste (most often).
I encourage anyone who is considering "giving up" to think about why. Give it the same level of consideration you gave starting. Treat it with the same concentration you do any other life altering decision, because I PROMISE YOU, giving up your passion will change your life.
Please excuse typos. I need an editor!