#1—Querying too soon.
Okay, I hate to say it, but it’s true. Most writers query too soon. In the past, books could be less ready to go and still get picked up by agents or editors, and there are plenty of agents who still do editorial before they send an author’s work out on submission.
Here’s the hard truth, though. Those days are mostly past. If we’re being honest, they only existed for a few lucky people anyway.
You’ll know if you’ve queried too soon by the feedback you get. Usually something about the story being confusing, not connecting with the characters, perhaps that the stakes aren’t high enough or don’t continue to rise. Maybe the plot sags in the middle or it takes too long to get going. If you get similar feedback from several agents or editors, then it might be time to take a look and see if what they are saying has some truth in it.
The good news is that you can probably do some work and try again. Lots of authors have stories of landing an agent or an editor after getting over fifty rejections. You can, too. Of course, there are no guarantees in life, or in publishing, but getting some “no” answers does not mean you’ll never get a “yes!”
Focus on your craft and make the book the best you can. Consider getting some editorial help from a reputable editor or coach, maybe find some trusty beta readers. If getting a traditional deal matters to you, do the work. If you are writing for fun, then enjoy the process and do what makes you happy.
Either way, understand why you write. Is it for you? Is it for readers? Is it something else? When you understand your purpose, it makes the bumps easier to take.