Today, you may believe a query letter and dealing with agents to be a waste of time and effort. I disagree. This is a first step and can feed you 'till you're fed up with criticism. No matter what I say, you will feel worse than a racoon crossing a busy highway with a crushed skunk in his path and a leviathan truck closing the gap. Which way do you go?
Plug that nose and keep trucking.
That means, no matter how many rejections you receive, take the feedback and keep on trying. This is assuming you have done your due diligence. "What due diligence?", you ask. Well, did you consider the feedback from unbiased beta readers? Did you have a professional editor and copy editor go through your work? Did you act on their feedback? Did you edit the work several times from differing perspectives?
If you did and you are comfortable with the concept that you have produced quality, then maybe your query letter is not relaying that concept.
A query letter is a professional document meant to be read by a professional. Yes, agents are just that and expect the same. The query letter must convey the theme of the story, introduce the protagonist and hint on the journey taken and the adversities encountered. That sounds simple enough, but... can you do all of that in 200 words or less?
Wow, that is where it takes imagination, hard work and most likely, the ultimate best writing skills you obtain.
Write your query letter and ask strangers to read it. When you get the twinkle in the eye, the upwards curl at the edge of the lip and the quick request to read the book, you are ready to submit to an agent. Just make sure you're gramer is prooper and their are no simpletown errs.