Querying can mess with your mind. It can crush your confidence. It can halt your productivity. And when you get a request or a "I love this" it can make the world sunshine and roses and rainbows. Like I said, it messes with you mind.
First of all...try really hard not to let it. Stand up straight. Shoulders back. Query with confidence.
Rejection is going to be a part of this. Know it. Own it. Your query is going to get rejected--at least once. Or if it doesn't, your book is going to get rejected by an editor as some point. No one makes it through it unscathed. Even J.K. Rowling had rejection. Realize that it's not you. Be okay with it. Or don't query.
Getting an agent isn't the end of the journey. It's really just the beginning. It doesn't mean a sale, it doesn't mean you've made it. It means you're one step higher on the ladder, and you're learning. Remember to not stop learning.
Sub it Club link on how to handle outstanding queries when you get an offer.
Post from Dahlia Adler about being back in the trenches AFTER separating from an agent
Agents are people, too. Which sounds dumb, I know. I got a very nice pass yesterday from someone who said my book was right up her alley. BUT, she's swamped with work and she's getting married in a month. Who knows, in another time and space, she might have loved my book and offered representation. In this life, it's "no thank you." This has NOTHING to do with me, and I can't do anything about it. So, say congratulations and move on.
1. Be polite
2. Be professional
If you can't do either of those things, you have no business querying or really interacting with anyone. This is a BUSINESS. You must know that going into this. You may be passionate about your work--and you should be--but don't let your passion made you blind to professionalism.
Seriously, if you can't be professional, no one will want to work with you. And that means twitter, goodreads, your blog, and in person. This is a small industry, really.
Good luck out there!