Myth #1: Do Not Self-Publish Before You Are Ready
The problem with this advice is that you are never ready and you'll always make mistakes. Self-publishing and writing are crafts you can never master. You have to improve and innovate yourself all the time. Even if you make it to the top of your game (whatever that means), you still need to try new things and get better if you want to stay where you are.
So you are basically never ready and you never fully know what you are doing.
I changed my book cover. I found mistakes and messed up formatting for books I self-published. Was it the end of the world? No. I improved those things. I'm getting better at what I can with the resources that I have.
It's better to try, fail, and learn than do nothing.
Also, the first book I ever self-published was Medusa's Desire, but it wasn't the first novel I ever wrote. While I have no wish to self-publish the previous two books I wrote, there's nothing tangible that I can say that explains why this book seemed like the right one to me. It just did.
Myth #2: Self-Published Authors Who Have Grammar or Spelling Mistakes In Their Novels Weren't Ready To Self-Publish
People get really mad at self-published authors with mistakes in their novels or self-published authors who have "poorly written" novels, whatever that means. The truth is, none of us can write a good novel. We need critique partners and editors. And even when we have those, there are still sometimes mistakes in our novels.
It's true with traditionally published novels as well. Some have small mistakes like typos and others have huge ones like poor character development. But that doesn't matter and doesn't always make it impossible for them to succeed.
It's best to minimize those mistakes, but it's also good to remember, that authors are humans, not gods. So we make mistakes and it's okay.
Also, people disagree on what makes a good novel. I know some people would say a novel riddled with grammar mistakes is automatically a terrible one, but not to everyone. When I was a teen, I preferred fan-fiction online riddled with mistakes over reading published and polished novels. *shrugs* So I liked the ones with mistakes better at the time.
And I've read at least one editor who said it's better to make grammar mistakes and have something interesting to write with good pacing and character development, then to have a story that's boring with terrible pacing and characters, but with zero grammar mistakes.