Creating believable characters takes time and thought. Will you keep repeating mistakes? A simple example is a character who is a mother who feels inadequate, who is not a super mom, who doesn't always know the right thing to do or say as a parent.
Be honest about the things that bother you. And why?
You will typically have between main characters and a handful of minor supporting characters in your book. Your book should be just as diverse as the real world.
What are the relationship dynamics between each character? Even if it is a topic you know a lot about, you should still try to research and fact-check just to make sure you have accurate information.
Then write about a character who shows those traits. That said, once you have written say 10, words of your first draft — STOP.
But that doesn't mean that their external characteristics should just be an after-thought.